When Katherine Russell arrived as a freshman at Suffolk University just over five years ago, she seemed to bond so well with her new roommates in their lively dorm opposite Boston Common that one classmate likened them to sitcom characters. "They reminded me of the show ‘Sex and the City,"’ he recalled. "Two of them were free-spirited, one was materialistic and Katherine was the social butterfly."

Then Russell began dating Tamerlan Tsarnaev, a boxer from Cambridge known for his flashy clothes, and her life began to change. As he became a steadily more religious Muslim, Russell converted to Islam. She started to cover her head with a hijab in public, startling some classmates. She dropped out of college in 2010, the year they got married and had a daughter.

She moved into his family’s run-down apartment in Cambridge, trading her old life of New England comfort and privilege — her father and grandfather both went to Phillips Exeter Academy and Yale — for the struggles of an immigrant family, with money so tight they were on public assistance at times.

Now Russell, 24, is known around the world as the widow of the man suspected of bombing the Boston Marathon with his brother before he was killed April 19 after a shootout with the police. And she has attracted the interest of the FBI, which is trying to determine whether she knew about the bombings or helped the two brothers in any way, knowingly or unknowingly, before or after the attacks.

The surviving bombing suspect, Dzhokhar Tsarnaev, 19, has told investigators that he and his brother built their bombs in the Cambridge apartment where Russell lived with Tamerlan, 26, and their daughter, Zahira, a toddler, according to two law enforcement officials. Other officials raised the possibility that the bombs may have been assembled elsewhere.

Investigators are also interested in a text message Russell sent to her husband after the FBI released photographs of him and his brother a few days after the bombings, two other law enforcement officials said. (This week, the FBI took samples of Russell’s DNA, and determined that her fingerprints and DNA did not match samples found on some bomb fragments, the officials said.)

Russell’s lawyers issued a statement saying the marathon bombings had "caused profound distress and sorrow to Katie and her family" and adding that "the reports of involvement by her husband and brother-in-law came as an absolute shock to them all."

The recent turn of events has stunned Russell’s friends, relatives, former classmates and neighbors. In North Kingstown, R.I., where she grew up, a newspaper, The Standard-Times, summed up local sentiment in a front-page headline. "NK native widow of Boston bombing suspect," it read. "Former high school classmate calls situation ‘odd."’

Russell grew up in a comfortable home on a leafy street there, the daughter of a doctor. Stephen Constantine, 23, who, like Russell, played alto saxophone in a middle school band, recalled her as popular and a good musician. "She could play more complex music than I could and learn it faster, and her sound was warmer and fuller bodied," he said.

In high school, she won an award for her drawing of a cat menacing a mouse. "It was a large colored-pencil drawing of a black cat with its paw raised and a gray mouse scooching out of the way," her art teacher, Amos Trout Paine, recalled. She quoted a David Bowie song, "Quicksand," in her high school yearbook.

Shortly after graduating, she had a brush with the law: She was arrested and charged with shoplifting five items worth $67 from an Old Navy at the Warwick Mall, according to a police report. She performed community service and paid money toward a general restitution fund that benefits crime victims, and the case was dismissed. The lawyer who represented her, J. Patrick O’Neill, who serves in the Rhode Island House of Representatives, said he could not recall details of the case or much about Russell.

In 2007, she moved to Boston to major in communications at Suffolk. It was there that friends introduced her to Tsarnaev, who had gone to a nearby community college. They dated on and off, people who knew them said, and eventually Russell converted to Islam.

She seemed to embrace her new religion willingly and enthusiastically, said someone who occasionally attended Russell family gatherings, and who spoke on the condition of anonymity so as not to betray the family’s confidence. "She was infatuated with this guy, and she adopted that religion," the person said, recalling a dinner in Boston when she announced that she had decided to start wearing a head scarf as part of her faith. "It was a big surprise."

Tsarnaev had a rough side: a domestic violence complaint was lodged against him in 2009 by another girlfriend, officials said. His father, Anzor Tsarnaev, said last month that he had "hit her lightly."

But things seemed promising for the young couple in 2010, said Julian Pollard, 31, a boxer who recounted a conversation with Tsarnaev that year at a Golden Gloves tournament in Lowell, Mass. "He said the training was going great, that he was happy with his faith and that he had just met a girl and he was very happy about that," he said. "He told me that he was engaged to her, or was about to propose."

They married on June 21, 2010, in a 15-minute ceremony in an office on the third floor of the Masjid Al Quran, on a quiet residential street in Dorchester. Imam Taalib Mahdee said that he had not met the couple before the ceremony, and that she was the one who had called and asked to be married there. "They were a happy couple," he said. Their marriage certificate listed his occupation as driver, hers as student.

But Russell did not go back to school that fall. Tsarnaev, who had given up boxing after being barred from national Golden Gloves tournaments because he was not a U.S. citizen, was growing increasingly religious, neighbors said. Money was scarce: the family’s income was supplemented by public assistance and food stamps from September 2011 to November 2012, state officials said. And last year, Tsarnaev left his wife and daughter behind in Cambridge for six months while he traveled to Dagestan to see his father, and to visit Chechnya.

Her mother-in-law, Zubeidat Tsarnaeva, said in an interview that Tsarnaev had wanted Russell and their child to move to Dagestan with him, and that she had been thinking about it. "She herself agreed, she said she wanted to study a different culture, language," Zubeidat Tsarnaeva said.

At times Russell supported the family by working as a home health aide — "working long hours, caring for people in their homes who are unable to care for themselves," her lawyers said in the statement.

One neighbor said Russell often seemed shy and quiet in the presence of her husband, but warmer and friendlier when he was not around. Another neighbor recalled hearing yells coming from the apartment.

A relative said Russell attended family gatherings less frequently, and withdrew a little from her old social life. "I think she believes in Islam," said the relative, who spoke on the condition of anonymity, and who said that she had seemed happy with her husband. "I don’t think she was coerced. I think she’s faithful to the religion."

On April 19, as the news spread that the Tsarnaev brothers were believed to have committed the marathon bombings and gone on a nightlong crime rampage that involved the fatal shooting of a campus police officer at the Massachusetts Institute of Technology, a carjacking, and a gunfight with the police in which Tamerlan was killed, a neighbor knocked at the door of their third-floor apartment in Cambridge, where Russell had apparently just heard the news from a relative. "She was in utter shock," the neighbor said. "Utter shock."

Then law enforcement officers arrived and ordered them all out. Their downstairs neighbor Albrect Ammon, 18, said Russell, who was dressed all in black, tried to borrow a cellphone from another woman, but an officer snatched it away, saying she was the suspect’s wife.

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