But a jury in Colorado didn't buy that claim, finding him guilty of first-degree murder.

Federal prosecutors accused Henthorn of pushing his second wife, Toni Henthorn, off a cliff in Rocky Mountain National Park in September 2012 to get millions of dollars in insurance money, CNN affiliate KDVR reported.

Authorities have also said they're re-investigating the 1995 death of Hanthorn's first wife, Sandra Lynn Henthorn.

At the time, Henthorn claimed his wife was changing a tire when the car fell on top of her. No charges have been filed in that case. But prosecutors presented details about it in court during his trial for Toni Henthorn's killing, arguing that there were troubling similarities. In both cases, Henthorn was the only witness.

Henthorn's attorney, Craig Truman, argued that both deaths were an accident.

"Today's conviction means that Harold Henthorn will never hurt or kill another woman," Bob Troyer, an assistant U.S. attorney, told reporters after the verdict was announced Monday. "Instead, he will likely spend the rest of his natural life in a prison cell."

Henthorn's conviction carries a mandatory life sentence, Justice Department spokesman Jeff Dorschner said. He is set to be sentenced on December 8.

Some of Toni Henthorn's family members sobbed Monday as the jury's verdict was announced, KDVR reported.

Her brother, Todd Bertolet, told CNN last year that the family immediately grew suspicious about her death because Henthorn kept changing his story about what had happened.

"I think there are four or five versions of the story he told," Bertolet said.

Bertolet said it wasn't until after his sister's death that his family learned the details surrounding the death of Henthorn's first wife.

"We knew he had been married previously. But he told us that she had died during a car accident. There's a big difference between a car crash and a car falling on you," Bertolet said. "I think if my sister had known the complete background on this guy, she never would have been with him."

CNN's Sara Weisfeldt contributed to this report.