The Spokane Valley Sheriff's Office has been investigating the murder of Montana Senator Jon Tester's nephew, Robert, since his brutal slaying on September 6.

Information Officer Deputy Mark Gregory said detectives uncovered an important lead when a friend of the suspect, John Radavich, contacted their office with a startling revelation.

"The last week of 2016, detectives investigating Mr. Tester's homicide received information from a friend of Mr. Radavich, the person we arrested on Thursday night," Gregory began. "That friend got a phone call from Mr. Radavich where he confessed to killing Mr. Tester. He was able to provide us with an audio recording of that phone call, and detectives were able to use that phone call in addition to physical evidence obtained at the scene and they were able to get a warrant for his arrest, at which point they went to his apartment and took him into custody."

Gregory said Radavich is the former boyfriend of a 17 year-old female who told him that Tester had assaulted her on September 5, the day before Tester was murdered.

"She went to Mr. Radavich and explained that she had been assaulted by Tester, and subsequently Mr. Tester was murdered the next day," he said.

According to court documents, the crime was particularly brutal and that the killer came armed with an ax and a knife. Tester's body was found in a hallway in a pool of blood.

Gregory said Radavich has been charged with first degree murder, that in Washington State might also qualify for the death penalty.

"With first degree murder he could receive a sentence of life imprisonment," he said. "If prosecutors decided to look into the possibility of the death penalty, that could also be an option, but that would be up to the prosecutor's office on what to ask for in sentencing."

Gregory said detectives are also looking into whether or not Radavich may have had accomplices in the brutal slaying.

"I'm not sure, nor am I at liberty to say, whether or not detectives are looking into any other assistance that Mr. Radavich may have had during this case," he said. "They are looking at all options and are continuing to process evidence."