EXCLUSIVE: Boston Marathon Bomber Tamerlan Tsarnaev 'Killed Three Men in Drug Deal Gone Bad' Years Before He Would Set Off Explosion and Take Multiple Lives In Horrific Tragedy

Boston Marathon bomber Tamerlan Tsarnaev may have taken numerous lives years before he and his younger brother, Dzhokhar, would set off a bomb that rocked the city.

On April 15, 2013, the siblings planted two pressure cooker bombs near the finish line of the marathon, leading to the deaths of three people and injuring hundreds more – but this may not have been Tamerlan's first taste of murder, RadarOnline.com can reveal.

Two years before the horrific tragedy, investigators believe Tamerlan – who was trained in boxing and mixed martial arts – was one of two men involved in the disturbing murders of Brendan Mess, 25, Erik Weiss, 31, and Raphael Teken, 37, at a Waltham, Massachusetts home in a drug deal gone bad.

Just shortly after Tamerlan's death – which occurred while on the run with his brother following the bombing – he was pointed out by his sparring partner Ibragim Todashev.

According to Todashev, Tamerlan was with him when he slashed the three men's throats, as investigators revealed the "heads of the three were pulled back and their throats slit ear to ear with great force."

CNN reported "marijuana was spread over the (dead) bodies and in a 'symbolic gesture,' several thousand dollars ($5,000) in cash was found at the scene." The men's bodies had been discovered by a girlfriend.

According to a law enforcement source, "the men were killed during a drug rip-off because he and (Tamerlan) were afraid they would be able to identify them and tell police."

Police also believe Mess was dealing drugs from the home. Since there was no sign of forced entry, authorities think the three victims knew the killers.

While friends reportedly identified Tamerlan as a friend of Mess, who also trained with him at the same gym, he was never questioned in the case by cops.

A pal of Mess claimed the "police wanted to know who all the friends were in the group, and I told them about Tamerlan."

Todashev made the allegations while being questioned by the FBI for nearly eight hours for his connections to Tamerlan. However, during the lengthy interview, Todashev reportedly attacked agent Aaron McFarlane with a weapon – said to be a pipe or stick – and was fatally shot.

The autopsy reported revealed the 27-year-old was shot seven times.

Dzhokhar, now 31, is currently on death row at ADX Florence in Colorado, best known to be one of the most secure prisons in the world, for his role in the Boston bombing.

Oklahoma City bomber Timothy McVeigh was once incarnated in the same prison before being transferred to USP Terre Haute in Indiana, in 1999 to be executed.

According to sources, McVeigh was a blubbering mess during his final days alive.