A federal grand jury asked the wife of dead Boston Marathon bombings suspect Tamerlan Tsarnaev for items that belonged to one of the victims in a triple slaying in Waltham three years ago, her attorney said. Federal prosecutors in Boston revealed in court documents that a friend of Tsarnaev told authorities that he and Tsarnaev participated in the killings in Waltham. Tsarnaev was killed in a confrontation with police after the bombing. The attorney for Tsarnaev’s widow, Katherine Russell, said a grand jury asked her for items that belonged to Brendan Mess, one of the three people killed in the Waltham slayings. Attorney Amato DeLuca said Russell does not have anything that belongs to Mess.