The officials said State Attorney Jeff Ashton will not bring charges against the agent.
Both officials spoke today on condition of anonymity because they weren't authorised to speak publicly about the case. Ashton's report will be released Tuesday. The Washington Post first reported the prosecutor's decision.
Ashton's office did not immediately respond Friday to calls and emails seeking comment.
Ibragim Todashev, a 27-year-old mixed martial arts fighter, was killed in Orlando in May while FBI agents and Massachusetts state troopers questioned him about his friendship with suspected Boston Marathon bomber Tamerlan Tsarnaev.
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Officials originally said Todashev lunged at an agent with a knife while he was being questioned. They later said it was no longer clear what happened.
Federal prosecutors have said in court filings that Todashev named Tsarnaev as a participant in an earlier triple homicide in Massachusetts. The filings were made in the case against Tsarnaev's brother, surviving bombing suspect Dzhokhar Tsarnaev.
In that case, three men were found in an apartment with their necks slit and their bodies reportedly covered with marijuana. One of the victims was a boxer and friend of Tamerlan Tsarnaev.
The filing was prosecutors' attempt to block Dzhokhar Tsarnaev from getting certain information from authorities, including investigative documents associated with the Waltham slayings.
Authorities allege that Dzhokhar Tsarnaev, 20, and 26-year-old Tamerlan Tsarnaev, ethnic Chechens from Russia, planned and carried out the twin bombings near the finish of the marathon on April 15. Three people were killed and more than 260 were injured.