(CNN) -- The in-laws of late Boston Marathon bombing suspect Tamerlan Tsarnaev appeared Thursday before a federal grand jury in Boston, a family attorney said.
Joshua Dratel -- the lawyer representing Katherine Russell, who was married to Tsarnaev before he died following a police shootout days after the deadly April bombing -- said that the widow's parents answered questions for four hours Thursday from the grand jury.
But, citing grand jury secrecy rules, Dratel declined to elaborate further about what the Russells testified about.
Christina Sterling, spokeswoman for the U.S. Attorney's Office for Massachusetts, said only that she "cannot confirm or deny the existence of a grand jury."
Four people have been charged in connection with the Boston attack, including Russell's brother-in-law, Dzhokhar Tsarnaev, who is suspected of carrying out the attack along with his brother, Tamerlan, 26.
Neither Katherine Russell nor her parents have been charged, and investigators haven't said what they believe she knew about the twin April 15 blasts that killed three people and injured more than 260 others.
A lawyer representing Russell, Amato DeLuca, said eight days after the bombing that Tamerlan Tsarnaev's widow was "doing everything she can to assist with the investigation." DeLuca said that Russell, then 24, knew nothing about any plan to plant explosives near the Boston Marathon's finish line and that reports her husband was involved came as an "absolute shock" to her and her family.
A 2007 graduate of North Kingstown High School in Rhode Island, she moved that year to Boston -- where she met Tamerlan Tsarnaev at a nightclub -- and enrolled at Suffolk University.
Russell dropped out of college her freshman year. Eventually, she converted to Islam before the couple married in a Massachusetts mosque in June 2010.
Together, she and her husband had a daughter, who was 3 at the time of the Boston bombing. Tamerlan Tsarnaev reportedly stayed home to care for the girl while Russell worked long hours as a home health aide.
CNN's Greg Botelho contributed to this report.