Confessed serial killer Israel Keyes admitted he enjoyed killing people, but couldn't or wouldn't give investigators a more meaningful answer when quizzed why he did it.

"There were just times, a couple of times, where we would try to get a why," said Anchorage Police officer Jeff Bell, who helped interrogate Keyes for hours.

"He would have this term, he would say, 'A lot of people ask why, and I would be, like, why not?"' Bell said.

Keyes confessed to killing eight people across the United States, but alluded to additional murders, FBI Special Agent Jolene Goeden and Bell told The Associated Press.

"Based on some of the things he told us, and some of the conversations we had with him, we believe the number is less than 12," Goeden said. "We don't know for sure. He's the only one who could have ultimately answered that."