Last month, CNN reported that Plotnikov and six others died in a July 2012 firefight with Russian forces in the southwestern republic of Dagestan, while Tsarnaev was visiting the region, according to a source briefed on the investigation.
Todashev, 27, knew Dzhokhar Tsarnaev, also a suspect in the April 15 bombings, the official said. Dzhokhar Tsarnaev, injured and captured after a manhunt, is being held by authorities. His brother died in a shootout with police.
Todashev was from the Chechnya region, as were the Tsarnaev brothers, the source said.
Todashev was granted political asylum in 2008, but he came to the United States some time before that, a federal law enforcement official told CNN. Todashev had been living in the United States as a legal resident because of that asylum claim.
In the 2011 Massachusetts triple homicide, the Middlesex County district attorney's office said at the time that the victims and two unknown perpetrators appeared to know each other and that it was not a random crime. No suspects were named then.
A source said that the FBI had been investigating Todashev for about a month.
The FBI had followed Todashev for days, his friend told CNN affiliate Central Florida News 13.
Todashev "wasn't like real close friends (with Tsarnaev), but he just happened to know him," Khasuen Taramov told the TV station. "But he had no idea that they were up to something like that, like bombings and everything, you know what I mean?"
He told CNN affiliate WESH that Todashev and Tsarnaev had spoken by telephone about a month before the bombings.
"It was a complete shock to him," Taramov said.
The two met a couple years ago in Boston, where Todashev had lived and where there is a small, close-knit community of Chechens, said Taramov.
Their telephone conversation before the bombings contained nothing but routine pleasantries, he said. "It was 'How are you doing; how's your family?' That's all."
Taramov said he himself was questioned by the FBI for three hours Tuesday night. Asked what he was asked, Taramov said, "Different kind of questions like 'what do you think about bombings,' 'do you know these guys,' blah blah blah, what is my views on certain stuff."
He said Todashev was not a radical. "He was just a Muslim. That was his mistake, I guess."
Taramov said his friend had told him he had a bad feeling about the direction the investigation was heading.
"He felt like there's going to be a setup ... bad setup against him. Because he told me, 'They are making up such crazy stuff, I don't know ... why they doing it. OK, I'm answering the questions, but they are still making up some, like, connections, some crazy stuff. I don't know why they are doing it.' "
Before meeting with the FBI for a 7:30 p.m. interview Tuesday, Taramov said, his friend asked him to take his parents' telephone numbers. "He just told me, 'Take the numbers, in case something happens, if I get locked up, or whatever, call them.' You know what I mean?
"We were expecting to get him locked up, but not getting him killed. I can't believe it."
Todashev was unemployed and had been living on insurance money he received after surgery for an accident. "He used to be a fighter, MMA fighter," Taramov said, in a reference to mixed martial arts.
Todashev was arrested this month on a charge of aggravated battery after getting into a fight over a parking spot with a man and his son outside an Orlando mall. The son was taken to a hospital with head injuries, a split upper lip and several teeth knocked out of place, the Orange County Sheriff's Office said in a report.