A witness who was in close contact with the Tsarnaev brothers just days prior to the Boston Marathon bombing is missing.

NBC News reports one of Dzhokhar Tsarnaev's lawyers informed the judge that Mogomed Dolakov, a Russian immigrant, is missing on Wednesday during a sidebar conversation in the death penalty trial. He told the judge that the defense team and the government cannot locate Dolakov.

Dolakov was interviewed by the FBI many times since the 2013 bombings.

"Maybe he just wants to move on with his life," Miriam Conrad, the defense lawyer, said of the witness, according to a transcript of the sidebar NBC News has.

The jury did not hear the conversation.

Dolakov became acquaintances with the Tsarnaev brothers while living in Cambridge, Mass. He told the FBI that when he first met the oldest brother, Tamerlan Tsarnaev, at a mosque, it was clear that he was a "radical" Muslim.

Dolakov also told agents that Tamerlan deemed the death of a police officer by way of a suicide bomb acceptable when it came up in conversation after they saw it in the news, according to an FBI report. Tamerlan said the cop in that situation was an infidel.

Three days before the bombings, Dolakov noticed that the car the Tsarnaev brothers rode around in had multiple items in it that were "covered by a white sheet," according to NBC News. He also told the FBI that Dzhokhar Tsarnaev said he was sick of living in the United States and wanted to go to Russia.

None of the FBI reports were sent in by the defense as evidence, so the information cannot be used in the trial. The defense, based on how Dolakov  painted Tamerlan as the mastermind of the bombings, planned on calling him to the stand to strengthen their argument that Dzhokhar was heavily influenced and under the control of his older brother leading up to the attacks.