ORLANDO, Fla.—One of the brothers accused of carrying out the Boston Marathon bombing is also now believed to have committed a grisly triple murder in 2011, U.S. law-enforcement officials said after a man believed to be his accomplice was gunned down here Wednesday by a federal agent who was questioning him.

Ibragim Todashev was shot dead at an Orlando apartment after he lunged at and cut a Federal Bureau of Investigation agent, who with two Massachusetts state troopers had been questioning him about the murders in Waltham, Mass., two law-enforcement officials said, citing preliminary reports from the scene. The agent was taken to the hospital with injuries that weren't life-threatening. The FBI said it was reviewing the incident.

During questioning, the 27-year-old Mr. Todashev made a series of incriminating statements about the murders, the officials said. They said that Mr. Todashev's remarks weren't a full confession but that he implicated himself and Tamerlan Tsarnaev, the bombing suspect who died after a shootout with police.

Representatives for the Middlesex District Attorney's Office, which is overseeing the homicide investigation, and the Massachusetts State Police declined to comment. The local probe is continuing, and investigators haven't determined whether other suspects were involved, the U.S. officials said.

The trail looked to have gone cold in the killings of Brendan Mess, 25, Erik Weissman, 31, and Raphael Teken, 37, whose throats were slashed on Sept. 11, 2011, in a Waltham apartment. Their bodies were found a day later, covered in marijuana and cash. No arrests were made, but the probe gained steam in recent weeks.