The man was identified as Ibragim Todashev, 27, a former Boston-area resident who knew Tsarnaev from martial-arts and boxing circles. Todashev had recently moved to Orlando from Cambridge, Mass.
Law enforcement officials said Todashev was not a suspect in the bombing. Instead, they said, he was being interviewed about his possible role in a triple murder in Waltham, Mass., on Sept. 11, 2011. They said Todashev acknowledged involvement in the murders and also implicated Tsarnaev in what the law enforcement officials described as a drug deal that went bad.
Dozens of FBI agents and other law enforcement officers have been conducting interviews across the United States and in Russia with associates of Tsarnaev and his brother, Dzhokhar, to learn whether anyone else was associated with the Boston attack.
The interviews have focused largely on people from the northern Caucasus area of Russia, where Tamerlan Tsarnaev spent six months in 2012. The Tsarnaev family has roots in Chechnya, part of the restive region, and the FBI suspects he might have had contact with Islamic extremists there last year.
Reached Wednesday in the Russian republic of Dagestan, Anzor Tsarnaev, Tamerlan's father, said his son knew Todashev and he believed the two had met while in the United States.
"All Chechens are like relatives," the elder Tsaraev said when told about the shooting in Florida. "This is all so sad."
He questioned how Todashev could have been killed during an interrogation. "Why do they think they have the right to kill people?" he said. "It was a set-up, like with Tamerlan."
Tamerlan Tsarnaev was killed in a shootout with police four days after the bombing. His brother was captured later that day. Before he was charged, Dzhokhar Tsarnaev told the FBI that no one else was involved in the plot.
Shortly after midnight Wednesday, the FBI agent, two Massachusetts State Police officers, and other law enforcement personnel went to Todashev's apartment in a residential area near Universal Studios in Orlando to interview him about his relationship with Tsarnaev and their suspected ties to the Waltham murders.
The officials said he was initially cooperative and appeared to be on the verge of signing a confession when the interview turned into a confrontation. The FBI agent sustained minor injuries and Todashev was shot and killed.