The FBI determined the man who opened fire on a crowd of 350 at an Orlando LGBT nightclub Sunday likely made up claims that he had ties to Boston Marathon bombers Dzhokhar and Tamerlan Tsarnaev.

Omar Mateen, 29, carried out the worst mass shooting in U.S. history early Sunday morning at the Pulse nightclub, killing 49 people and injuring more than 50 others. Mateen, who was shot and killed by law enforcement during a standoff at the club, had previously been investigated for claiming to have ties to the Tsarnaev terrorists. 

Related:  What we know about the Orlando gunman

But the FBI probed Mateen's claims in 2013 and found that he had no such ties, according to federal investigators.

Special Agent in Charge Harold H. Shaw sent out a statement Monday saying, "During one of the 911 calls between the operator and Omar Mir Seddique Mateen, Mateen made a reference to the Tsarnaev brothers by calling them his 'homeboys.' At this point in time, all evidence collected to date shows no connection between Mateen and the Tsarnaev brothers."

The FBI also confirmed Mateen’s false claim to The Guardian, but would not comment on the invented Tsarnaev connection.

“I can confirm that the subject made allegations of terror ties that investigation proved to be unfounded, but I can’t comment on the specific association,” Matthew Berton, an FBI spokesman, told the publication.

The Tsarnaev brothers carried out the April 15, 2013, terror attacks at the Boston Marathon that killed three people, including an 8-year-old boy, and injured more than 260 others. The case was regarded as an act of homegrown terrorism, though the brothers claimed allegiance to Islamic extremists. 

Before Sunday's deadly rampage, Mateen called 911 to proclaim his allegiance to the Islamic State (ISIS), a terror organization known for carrying out widespread acts of terrorism and brutal murders. ISIS quickly claimed credit for Sunday's massacre, though U.S. officials have said they consider Mateen a "lone wolf" attacker with no ties to any terror organization.

Many reports claim Mateen had a history of homophobia and abusive behavior. His father, Seddique Mir Mateen, said his son became enraged after seeing two men kissing in Miami months ago. 

Mateen “saw two men kissing each other in front of his wife and kid and he got very angry,” he told NBC News.

Shaw said feds had no specific, credible threats to the Boston area as a result of the Orlando attack, but asked the public to remain vigilant.

"As always, the FBI Boston Division will work with our law enforcement partners to gather, share and act upon threat information as it comes to our attention. We urge the public to report any and all suspicious activity to law enforcement," Shaw said.