Law enforcement officials have told ABC News that "mounting" forensic evidence from an unsolved triple homicide ties Tamerlan and Dzhokhar Tsarnaev to the crime.
Cell phone records also place the brothers in the area of the murders on the date they were committed.
On Sept. 11, 2011, three men were found in a residence with their throats slit and their bodies covered in marijuana.
Tamerlan described one of the murder victims, 25-year-old Brendan Mess, as his best friend. He and Mess were once roommates and boxed together. The two others who died were 31-year-old Erik Weissman and 37-year-old Raphael Teken, both of Cambridge.
After the killings, Middlesex District Attorney Gerard Leone released a statement saying "based on the present state of the investigation, it is believed that the victims knew the assailant or assailants, and the attacks were not random."
No one was ever arrested in connection to the murders.
The brothers are suspected of planting bombs at the Boston marathon in an attack that killed three people and injured more than 200 others. Authorities say Dzhokhar admitted to the bombing. Tamerlan, 26, died in a police shootout days after the attack.
Officials told ABC News that it's too early to bring an indictment against Dzhokhar. They are waiting on more definitive DNA testing.
Dzhokhar, 19, has been charged with one count of using and conspiring to use a weapon of mass destruction and one count of malicious destruction of property with an explosive device in connecting with the Boston bombings.
Tamerlan's body has been buried at a Muslim site in Virginia.
SEE ALSO: BOSTON MASSACRE: The Full Story Of How Two Deranged Young Men Terrorized An American City
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