An FBI agent shot and killed a man of Chechen origin who turned violent while being questioned today about his connection to Boston bombing suspect Tamerlan Tsarnaev.
The latter is one of two Chechen brothers suspected of carrying out the Boston Marathon attack.
A friend of the dead man identified him to local media as 27-year-old Ibragim Todashev, who had previously lived in Boston and knew Tamerlan Tsarnaev.
Tamerlan is the older of the two brothers suspected of planting two bombs at the marathon on 15 April.
Three people were killed and 264 injured in the attacks.
The shooting occurred in an apartment complex near the Universal Studios theme park, where the FBI and members of other law enforcement agencies including the Massachusetts State Police were interviewing the man about the marathon bombing.
"A violent confrontation was initiated by the individual," the FBI said.
A special agent, it said, "acting on the imminent threat posed by the individual, responded with deadly force.
"The individual was killed and the special agent was transported to the hospital with non-life-threatening injuries."
Authorities were also stepping up their investigation into possible connections between Tamerlan Tsarnaev, who died in a shootout with police, and an unsolved 2011 triple homicide in a Boston suburb that investigators believe was drug related.
NBC News reported today that the man killed in Florida had confessed to the FBI that he had played a role in the 2011 murders.
A US government official close to the investigation had no immediate comment.
Tamerlan Tsarnaev, 26, and his 19-year-old brother Dzhokhar are suspected of setting off two pressure-cooker bombs at the marathon finish line.
Dzhokhar Tsarnaev is being held at a prison hospital west of Boston awaiting trial on charges that carry the possibility of the death penalty.
The friend of the man who was shot today, Khusn Taramiv, said Mr Todashev knew Tamerlan Tsarnaev because both were mixed martial-arts fighters but had no connection to the bombing.
Mr Taramiv said Mr Todashev and others in the complex, some of whom were of Chechen origin, had been questioned several times by law enforcement agents since the day the Tsarnaev brothers were identified as the bombing suspects.
Law enforcement officials have also interviewed another person of Chechen origin, ex-rebel Musa Khadzhimuratov, at his home in New Hampshire, the New York Times reported last week.
Mr Khadzhimuratov had also had contact with Tamerlan Tsarnaev.
Law enforcement agencies are also looking for clues linking Tamerlan Tsarnaev to a 12 September 2011, triple murder in Waltham, Massachusetts, where three men including a close friend of Tamerlan Tsarnaev were found stabbed in the neck in an apartment.
News reports said that marijuana was strewn over their bodies.
The possibility that Tamerlan Tsarnaev was connected to the Waltham murders is "being looked at seriously," said Peter King, chairman of the counter-terrorism and intelligence subcommittee of the House Homeland Security Committee.
Other US officials confirmed the investigation did involve Mr Tsarnaev's possible role.
A source said, however, that while Tamerlan Tsarnaev's connection to the Waltham killings was actively under investigation, at this point there was no evidence to suggest the murders had any connection to the Tsarnaev brothers' possible motives or actions in allegedly carrying out the marathon bombings.
A spokeswoman for the office of the District Attorney of Middlesex County, Massachusetts, where the murders occurred, said that her office had been conducting an open and active homicide investigation in the case since 2011.
She said that local and state police investigators were involved.
She declined to comment on possible involvement of federal agencies.
Before the Boston bombings, Tamerlan Tsarnaev had been listed on multiple US government databases, including a master list of potential terrorism suspects.
US authorities also were asked twice by Russia to investigate Tamerlan Tsarnaev for possible involvement with Islamic militants, US officials have said.
Also, this afternoon, US Attorney General Eric Holder was scheduled to meet in Washington with Vladimir Kolokoltsev, Russia's interior minister.
The meeting's agenda was unclear, but US investigators are anxious to learn what Russian authorities knew about the Tsarnaevs and about what Tamerlan Tsarnaev did during a six-month trip to Russia last year.