The wife of the dead Boston Marathon bombing suspect, Katherine Russell, is assisting authorities with their investigation.

Her lawyer, Amato DeLuca, said she is in absolute shock that her husband and brother-in-law were accused of the deadly blasts.

"She cries a lot," Mr DeLuca said of the 24-year-old.

Ms Russell is an American-born convert to Islam who was married to Tamerlan Tsarnaev in June 2010.

Tamerlan Tsarnaev, 26, was killed in a shootout with police and younger brother Dzhokhar Tsarnaev, 19, lies wounded in a Boston hospital charged with using weapons of mass destruction in the twin blasts that killed three people and wounded 264 near the finish line of the Boston Marathon on 15 April.

The two ethnic Chechen brothers remain the only known suspects.

Dzhokhar Tsarnaev's condition improved to "fair" from "serious" as he recovered from gunshot wounds at Beth Israel Deaconess Medical Center, where in an impromptu hearing on Monday he was charged with two crimes that could result in the death penalty if he is convicted.

Ms Russell's lawyer called a news conference to deny that she had any connection to or knowledge of the bombings, saying she was busy caring for the couple's daughter and working as a home healthcare aide in the time leading up to the blasts.

"She is doing everything she can to assist with the investigation," Mr DeLuca said outside his office in Providence, Rhode Island.

"The reports of involvement by her husband and brother-in-law came as an absolute shock to them all."

Private funeral for 8-year-old victim

Meanwhile, the family of 8-year-old Martin Richard, the youngest person to die in the attacks, privately buried their son on Tuesday.

"This has been the most difficult week of our lives and we appreciate that our friends and family have given us space to grieve and heal," parents Denise and Bill Richard said in a statement. "We laid our son Martin to rest, and he is now at peace."

FBI faces questions for how it handled information

Russian authorities flagged the older Tsarnaev in 2011 as a possible Islamist radical, and some lawmakers have accused the FBI of failing to act thoroughly enough after Russia's security services raised their concerns with the United States.

The FBI questioned him in 2011.

At a Senate Judiciary Committee hearing on immigration legislation, Republican Senator Lindsey Graham of South Carolina asked Homeland Security Secretary Janet Napolitano why the older brother was not questioned upon returning from Russia in 2012.

Ms Napolitano said US Customs generated an alert when he left the country but neither Customs nor the Federal Bureau of Investigation was aware of his return six months later.

"The FBI text alert on him at that point was more than a year old and had expired," she said.

Ms Napolitano also dispelled reports authorities may have lost track of Tamerlan Tsarnaev because his name was spelled differently on an airline manifest.