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A 22-year-old man was arrested Thursday in the brutal September axe-killing of the nephew of Sen. Jon Tester, D-Montana, Washington state authorities said.

What authorities said for months was a slaying during a home invasion was actually a cold-blooded revenge killing of Bob Tester, 35, who was found lying in a pool of blood in a Spokane County home on Sept. 6, the county sheriff's office said. He died at the scene.

Bob Tester, the nephew of Sen. Jon Tester, D-Montana. via Facebook

John Arthur Radavich was arrested late Thursday afternoon at an apartment in the Spokane suburb of Spokane Valley. He was held on $1 million bond on suspicion of first-degree murder, according to court records.

The sheriff's office said a friend of Radavich's told investigators that Radavich admitted in a phone call to having killed the senator's nephew "because Tester was abusing his 17-year-old girlfriend and he had to take care of it by killing Tester."

The friend recorded the call and turned the recording over to detectives, the sheriff's office said.

Detectives learned that Tester had been dating Radavich's 17-year-old ex-girlfriend, the sheriff's office said. She contacted Radavich after she was assaulted by Tester on Sept. 5, it said.

No further information was provided Thursday night. No hearing was immediately scheduled, and no attorney for Radavich was listed in court records.