CHICAGO >> The patients have arrived at emergency rooms across Illinois with mysterious, unnerving symptoms. Some were vomiting blood or bleeding from their gums and noses. Others had arms and legs that were mottled purple, splotched with unexplained bruises. Many were young and previously healthy, panicked and confused about what was happening to them.

Health officials say the patients are believed to have inhaled synthetic marijuana, otherwise known as Spice or K2, laced with a pesticide called brodifacoum that is used as rat poison and causes severe bleeding. The outbreak has sickened 95 people in Illinois, Wisconsin and Indiana; two of them have died.

A month after reports of symptoms began emerging, the outbreak has continued unabated. Hospitals have been reporting cases each day to Illinois’ health department, as federal and local law enforcement agencies search for sources of the tainted drugs. Synthetic marijuana, dried plant matter sprayed with chemicals and then smoked, is illegally sold in shiny foil packages with names like Insane, Crazy Monkey and Blue Giant from the counters of gas stations, minimarts and head shops.

“It’s been really kind of unbelievable,” said Dr. Steven Aks, an emergency medicine physician and the director of medical toxicology for the Cook County Health and Hospitals System. “These are folks who are using a recreational drug. I don’t think any of them are expecting it to turn into a bleeding problem.”

Outbreaks of illness related to synthetic marijuana have occurred in recent years in Lancaster County, Pennsylvania, where more than 150 people were sickened in one week, and in New York City in 2016, when 33 people reported falling ill. But state and national public health officials say this is the first time they have seen synthetic marijuana result in severe bleeding in users, a condition that if left untreated can be deadly.

An investigation by the Chicago Police Department and the federal Drug Enforcement Administration has resulted in three arrests so far; the suspects, charged with federal drug offenses this week, are believed to have sold at least some of the drugs from behind the counter of a minimart on the West Side of Chicago.

But officials here said they still have unanswered, vexing questions: How much of the tainted drug is still circulating throughout the Midwest, and how many more people will be sickened? Where are the drugs coming from, and how far have they spread? How did a dangerous chemical like brodifacoum, which does not create a sensation of being high, end up in synthetic marijuana?

“This is not something we have previously seen,” said Dr. Patrick Lank of Northwestern Memorial Hospital. “As someone who watches public health, I worry, why is this happening? How did a substance get into these drugs that absolutely has no high?”

Renee Funk, the associate director for emergency management for the Centers for Disease Control and Prevention, said the agency is sending a team to Illinois to assist with the investigation.

In Michigan, health officials issued a warning to the public this week, urging vigilance. On Thursday, the Wisconsin Division of Public Health reported that it had seen its first case in the state and was warning doctors in emergency rooms to be watchful for more.

“These drugs present a terrifying public health risk,” said Dr. Jon Meiman, chief medical officer with the Wisconsin agency. “We’re taking it very seriously.”

Synthetic marijuana is extremely lucrative for retailers, who sell it illegally, said Cara Smith, chief policy officer with the Cook County sheriff’s office. Truck drivers who are delivering bags of chips and sodas to gas stations and bodegas are frequently paid to distribute the drug, she said; customers pay about $10 for a 5-gram package.

Some people who are required to undergo routine drug testing use synthetic marijuana to get high because its components are often difficult to detect on tests.

“It’s a concoction of lots and lots of different chemicals,” said Dr. Adam Black, a physician at Presence Saints Mary and Elizabeth Medical Center in Chicago, which has treated several synthetic marijuana users with severe bleeding this week. “Even if patients are awake and self-reporting that they smoked this stuff, they don’t know what’s in it.”

The bleeding symptoms may not appear until days after the drugs are ingested, doctors said. Patients can be treated with vitamin K, which helps counteract the effects of the blood thinner.

Anthony Guglielmi, a spokesman for the Chicago Police Department, said that officers are looking for a motive and trying to determine whether any other stores are still selling the contaminated narcotics.

In a federal courtroom Tuesday, two of the men charged with drug crimes appeared before a judge, wearing bright orange prison garb. According to investigators, they both sold tainted synthetic marijuana at a Chicago convenience store. On a typical day, one of the suspects told investigators, the minimart sold close to 80 packages of synthetic marijuana. In recent weeks, customers had complained about the quality of the drug.

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