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 Ms. Russell began dating Tamerlan Tsarnaev, a boxer from Cambridge, Mass., known for his flashy clothes, and her life began to change. As he became a steadily more religious Muslim, Ms. 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 that they were on public assistance at times.

Now Ms. 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 F.B.I., 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.