Before he allegedly kidnapped Jayme Closs and killed her parents, Jake Thomas Patterson was already showing signs that he was deeply disturbed — including killing a dog and collecting dead animal heads, a new report says.

A young woman who says she dated Patterson for four months during their freshman year of high school told the website Radar Online that in 2011, a dead dog was found in some local woods.

“That’s when he told me he was ‘experimenting’ with animals. I was like, ‘What the f–k?’ ” said the woman, only identified as Briana.

She said the accused killer kidnapper also had a fetish for roadkill.

“It was an insane obsession of his,” she said. “His cabin was full of animal heads, but I don’t think people understand that he found those animals on the side of the road and stuffed them himself.”

She said she met Patterson in math class and thought that while “nerdy,’’ he also was “kind of cool’’ and began dating him.

But his inner “rage’’ quickly surfaced, Briana said.

“He’d lose his temper easily,” she said. “He was quiet, but when he got mad, he was a completely different person.”

Briana said that after one disagreement between them, she was terrified — when he suddenly appeared in a bizarre mask.

“I saw his true colors that night, he was so full of rage,’’ she said. “He literally scared the hell out of me. I was at the cabin, and he came out with this weird-looking masquerade mask. It was scary as hell.”

Even after Briana broke up with Patterson, he was still wreaking havoc on her, she said.

“He slashed my mom’s tires a few days after we broke up,” she said.

The woman said that when she heard what Patterson is accused of, “I dropped a glass I was holding. I said, ‘No. No way.’

“I mean, I wasn’t surprised, but I was. That could’ve been anyone — it could’ve been me. It could’ve been my sister. I slept at that house. I ate at that house. We all hung out at that house — it was beyond surreal.”