Joshua L. Dratel, a New York criminal attorney, joined the legal team representing Tsarnaev's widow, Katherine Russell, who hasn't been named a suspect nor charged in the investigation, the Providence (R.I.) Journal reported Wednesday.
Dratel said he would help Russell "navigate a system that is delicate, because of the nature of the situation."
Dratel agreed last week to assist lawyers Amato A. DeLuca and Miriam Weizenbaum of Providence. Russell grew up in North Kingstown, R.I.
Federal authorities are interested in what Russell, 24, may have known about her Tsarneav's involvement in the Boston Marathon bombings or the alleged involvement of his younger brother, Dzhokhar Tsarnaev.
Tamerlan Tsarnaev, 26, was killed in a shootout with police early April 19, four days after he and his 19-year-old brother allegedly detonated two bombs near the finish line of the Boston Marathon, killing three people and wounding more than 260 others. His brother was captured on April 19 and is in a federal prison medical facility in Massachusetts.