He was one of the most beloved teachers in the small world of international schools that serve the children of diplomats, well-off American expatriates and local elites. He was often the first to arrive in the morning, and the last to leave each day. He led students on class trips to exotic places, treating them to cookies and milk at bedtime.

That was the public persona of William Vahey, carefully crafted over four decades until a maid cleaning his home in Nicaragua stole a 16-gigabyte memory drive. There, in photograph after photograph, was evidence that the model teacher had molested scores of adolescent boys, possibly far more, in a four-decade career spanning 10 schools on four continents.

The discovery of a man the FBI regards as one of the most prolific pedophiles in memory has set off a crisis in the close-knit community of international schools, where horrified parents are being told their children may have been victims of a favorite teacher, and administrators are scurrying to close teacher-vetting loopholes revealed by Vahey’s abuses.

“With the sheer volume, the sheer number of incidents in which this man molested, it surprises me that somehow this was not picked up by someone,” said John Magagna, the founding director of Search Associates, the world’s largest international school recruiting firm. “I don’t know what went wrong.”

Apparently not even Vahey’s victims knew they had been molested. The double-cream Oreos that he handed out at bedtime on the overnight trips were laced with sleeping pills — enough to leave the boys unconscious as he touched them, and posed them for lewd photographs.

Vahey, a 64-year-old native of West Point, New York, attempted suicide in Nicaragua after his maid stole the drive. He survived, but killed himself on a second try, stabbing himself to death in Minnesota on March 21 and leaving hundreds of former students wondering if they were abused.

The agonized father of a student in Caracas, Venezuela, said his son, like many others, would rather not find out, but the boy cannot forget one fact. “He ate the cookies, too,” said the father, who spoke on condition of anonymity to protect his son’s identity. “Everyone on those trips did.”

There were decades of missed opportunities to expose Vahey. An early California sex-abuse conviction didn’t prevent him taking a series of jobs working with children. Colleagues and supervisors failed to question why he was so often with boys overnight. And at least twice, boys fell mysteriously ill while under his care and there was no investigation into Vahey’s role.

In 1969, Vahey, the son of a decorated World War II pilot, was arrested on child sexual abuse charges after police said he pinched the penises of eight boys, ages 7 to 9, at an Orange County high school where he gave swimming lessons.

Vahey, then 20, told authorities he had started touching boys without their consent at age 14, when he fondled a sleeping teen on a Boy Scout camping trip. He said he touched the genitals or anuses of sleeping boys four more times before the arrest.

The psychiatrist diagnosed Vahey with an “inadequate personality,” but added that the disorder did not predispose him to sexual offenses dangerous to others. The court even allowed Vahey to start work as a public school teacher’s aide after his arrest.

Vahey pleaded guilty to a single charge of lewd and lascivious behavior. He received a 90-day jail sentence and five years’ probation, with a condition that he should be supervised in the company of males younger than 16 during that time. After two years on probation, he was allowed to leave the country unsupervised following college graduation in 1972.

Such leniency was common at the time, said Dan Scott, a retired detective sergeant who worked for 26 years with the Los Angeles Sheriff’s Department office that investigates child abuse. “Nobody went after sex offenders.”

Vahey was required to register as a sex offender and update his address whenever he moved, but he never updated his information after the first time he registered and authorities didn’t pursue the matter. When the state registry was put online in 2004, his name wasn’t included because authorities discovered he was no longer living in California.

Vahey began his international teaching career with a year at the American School in Tehran in the run-up to Iran’s oil boom, the first in a series of stays around the Middle East and Europe. He taught history, social studies and related subjects in Lebanon, Spain, Iran again, Greece and then Saudi Arabia, almost always to middle school students.

By the time he arrived in Saudi Arabia, Vahey was married and had two sons with Jean Vahey, a woman who became a widely respected administrator in international education. He taught eighth- and ninth-grade social studies, coached boys’ basketball and led school trips to Bahrain, Turkey and Africa.

Halfway through his 12-year stay in Saudi Arabia, he received a principal’s certificate in New Jersey. It was March 1986, seven months before a law took effect requiring all new teachers and administrators to undergo background checks. New Jersey Education Department spokesman Mike Yaple said there is no record of Vahey undergoing a check before he got the certificate.

In Saudi Arabia, Vahey displayed an openness and concern for children that people would remark upon for the rest of his career, said Max Crum, a 38-year-old former student.

“When you’re a kid, and you have teachers who are stern and mean, you kind of fear them. He wasn’t that at all,” Crum said.

By 1992, Vahey and his wife moved to the prestigious Jakarta International School in Indonesia, where he again taught social studies, coached basketball and developed a reputation for leading fun overnight trips for students.

“All the kids seemed to really, really like him,” said Will Julius, a 21-year-old alumnus and son of a former longtime International School teacher.

After 10 years the Vaheys moved to Escuela Campo Alegre in Venezuela, where Jean became superintendent and the man known to all as Bill took a teaching job at the sprawling hillside campus overlooking the capital, Caracas.

All new hires were required to provide a police record from either their home of record or their last country of assignment, if they had been there for more than five years. Vahey presented one from Indonesia with no history of problems.

Again in Venezuela, the popular teacher and family man took students on trips

Authorities may have missed a warning sign when two students under Vahey’s care were rushed to a hospital after falling unconscious in their hotel room during a trip to a basketball game, parents and staff said. A toxicity test came back negative, so the school sent security officials to investigate. They were unable to determine a cause and chalked the incident up to a possible failing air conditioning unit. Vahey was not investigated.

Vahey also privately organized outings like a week-long annual trip to Costa Rica for some 20 to 25 students, parents and staff said.

Superintendent Gregory Hedger said that when he arrived he was surprised to learn such excursions were allowed. He put an immediate stop to trips not sponsored by the school and without parent chaperones.

Seven years later, the Vaheys went to work at the Westminster campus of London’s Southbank International School, which has about 350 pupils from 70 countries. Vahey was subjected to two criminal background checks in the United Kingdom, where he hadn’t lived before.

After a year, Vahey founded the “Travel Club,” according to a 2012 article in Focus, a magazine for expatriates in London. The school’s website describes a 13-day trip to Nepal in 2012 that included a trek in the Himalayas, white-water rafting and an elephant safari. In 2013, the spring break trip by students in grades six to nine was to Panama.

Southbank’s chair of governors, Chris Woodhead, told Britain’s Press Association there had been one complaint against Vahey. A boy on a trip felt sick, Woodhead said, and Vahey took the child into his room, apparently “to look after him.”

“A few months later a teacher heard gossiping on a minibus and the incident was investigated,” Woodhead said. “The boy’s parents agreed that there was nothing untoward and the matter shouldn’t be pursued.”

When Vahey went on to the American Nicaraguan School with glowing references, his wife stayed in London.

In Nicaragua, Vahey taught ninth-grade world history and advanced geography in such an exciting way that students asked why other teachers couldn’t do the same. He drew little notice from his neighbors until days before Thanksgiving.

That weekend, after Vahey’s maid was fired for stealing, he stopped leaving the house, said Rafael, a caretaker who declined to give his last name because of the sensitivity of the topic. All Sunday, the house was dark. On Monday, school employees who let themselves in with a master key found Vahey motionless in his bed.

Two groups of emergency officials declared him dead and called the coroner. The school’s director, Gloria Doll, sent teachers and parents a message saying Vahey “will be greatly missed, as he has truly been an integral part of our community.”

But as a black body bag was being unloaded, Rafael said, a police officer or paramedic let out a shout. Vahey was alive.

A doctor at the hospital where he was taken said he had a weak pulse and high levels of blood toxins, but could go home after two days. The doctor spoke on condition of anonymity because he was not authorized to discuss the matter.

“All we know is that he is still with us, and that is wonderful, miraculous news,” Doll wrote to the school later that day.

Vahey told colleagues he had been bitten by a poisonous spider. In early December, he sent a mass email message to the school, declaring that “it is with a sense of awe, wonder and joy that I find myself writing to you. The last week has been quite the adventure, and I am fortunate to say I am doing much better.”

He was headed to Atlanta for medical tests, Vahey wrote. “Get ready for an exciting second semester as I look forward to my return to the school in January.”

In early March, the maid reappeared and handed the USB drive to Doll, saying she should take a look. Its folders were marked with the names and dates of school trips dating to 2008: “Panama Trip,” ”Costa Rica Trip,” ”Basketball Trip” and “Spring 2013.”

Doll clicked on the last folder, where she found photos of unconscious boys, many blonde or red-headed and between the ages of 12 and 14. Some had their pants off, with a man’s hand touching their testicles or anus. Other boys were posed together in positions suggesting oral sex.

Doll confronted Vahey, who told her, according to an FBI affidavit, that he had given the boys sleeping pills, adding: “I was molested as a boy, that is why I do this. I have been doing this my whole life.”

Vahey said he had swallowed more than 100 sleeping pills in November after discovering the USB drive had been taken.

Doll demanded Vahey’s resignation, according to the affidavit.

Vahey flew to Atlanta the next day. It was only after he boarded that the school notified authorities at the U.S. Embassy in Managua, U.S. officials said. The U.S. officials immediately notified Nicaraguan police, but Vahey had left the country.

In Miami, where he changed planes, a special agent from the FBI notified Vahey of the investigation. But a bag check turned up nothing illegal, and the agents had no grounds on which to hold him.

By March 14, the school told parents Vahey had resigned. The teacher traveled to Luverne, Minnesota, where his brother, sister-in-law and mother live, the latter in a nursing home. He checked into a hotel where he fatally stabbed himself in the chest with a knife.

He left a note apologizing to his family.

“He’s one of the most prolific pedophiles that we’ve seen here due to the sheer numbers,” FBI Special Agent Shauna Dunlap said. “The number continues to go up. We continue to have folks reaching out to us.”

At least 60 of the 90 or so children in the images were from the Southbank school, according to police, where a significant number of parents said they did not want to know if their children were abused. Woodhead, the governor, has blamed the U.S. system.

“How did he qualify as a teacher in the United States, how is it this information was never available to any of the schools across the world who employed him over the next 40 years?” he asked in an interview with the Press Association.

In Venezuela, students are coming forward with details that suggest they may have been abused.

“We don’t know who was drugged. Right now we’re just listening,” superintendent Hedger said. “There’s an enormous sense of betrayal … He was one of the most popular, if not the most popular, teachers in the school.”

A woman answering a phone registered in Jean and Bill Vahey’s name hung up without speaking when called by an Associated Press reporter. His brothers and sons also could not be reached for comment.

Schools where Vahey taught are reviewing their background check policies and security procedures. Jane Larsson, executive director of the Council of International Schools, said a group of six international education associations was examining how schools could close loopholes allowing pedophiles to move from country to country without being detected by background checks or other reports.

“When this kind of thing happens it’s a shock to everyone and it mobilizes action,” she said.

Teacher recruiting firms are conducting a similar joint review.

In the meantime, one of the men molested by Vahey in the Westminster, California, swimming pool as a 9-year-old boy said that learning what had happened since then revived terrible memories.

“It certainly bothers me that a person like that would be left unsupervised and obviously not tracked over the last 45 years now,” the man said, his voice growing unsteady. “I find it troubling. I guess the question is: How can the system allow that to happen?”

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Weissenstein reported this story from Mexico City and Abdollah reported from Los Angeles. AP writers Luis Manuel Galeano in Managua, Nicaragua; Adam Schreck in Dubai; Niniek Karmini and Margie Mason in Jakarta; Joshua Goodman in Caracas, Venezuela; Jill Lawless and Sylvia Hui in London; and Carson Walker in Luverne, Minnesota, contributed to this report.