(CNN) -- The slaying of one of Tamerlan Tsarnaev's best friends in September 2011 is now being reviewed by a "wider group of eyes,"a source close to the Boston bombing investigation tells CNN.
The killings of Brendan Mess and two others -- all of whom had their throats slit -- in Waltham, Massachusetts, remain unsolved. The Middlesex County district attorney's office said at the time that the three victims were killed by "sharp force injuries of the neck."
District Attorney Gerry Leone said days after the crime that the victims and two unknown perpetrators appeared to know each other and that it was not a random crime. No suspects were named or arrests made in the case.
Tsarnaev and Mess were sparring partners at a local gym, and the source tells CNN investigators believe he was one of the last people to see Mess alive.
Investigators of the crime reported at the time that the heads of the three victims were pulled back and their throats slit ear to ear with great force. Marijuana was spread over the bodies in a "symbolic gesture," and several thousand dollars in cash was found at the scene.
On Monday, Middlesex County District Attorney's spokeswoman Mary Beth Long told CNN, "We will review any new information that may come to light in (the triple homicide) as a result of the investigation into alleged marathon bombing suspect Tamarlen Tsarnaev."
Long said the case has always been an open investigation and that they continue to follow all leads.
A sister of one of the victims told CNN that she has been recently contacted by investigators in the case.
"She's meeting with them next week," said CNN's Ashleigh Banfield. "Investigators on that case are meeting with her and have been speaking with her since Saturday. Not only that, she confirms to me that the same detectives are still on that case and that they are detectives that she has spoken with in the past."
The sister, who requested to remain anonymous, said investigators have been very interested in trying to determine if there is any relationship between her brother and Tsarnaev, Banfield said.
"She couldn't really enlighten these detectives at all because she felt her brother wasn't a friend or acquaintance at all of Tamerlan Tsarnaev," Banfield said.
But the sister said she hopes the new interest will lead to her brother's case being solved.