Mr. Clark’s family could not immediately be reached for comment.

Image Brandon Clark, in a photo posted to Instagram. Credit...

Officers arrived at the scene some time after 7 a.m. on Sunday after multiple phone calls, including one from Mr. Clark, the police said. In the call, he had “made incriminating statements” and alluded to wanting to harm himself.

When officers arrived, officials said, they found Mr. Clark lying down next to a black S.U.V.

As soon as they approached, he began stabbing himself in the neck. Shortly after, the police said, he laid his body across a green tarp that was covering something on the ground and began taking photos of himself. The officers later learned the tarp was covering Ms. Devins’s nearly decapitated body.

Screenshots show that Mr. Clark posted photos of himself to Snapchat in which he was bleeding from a neck injury and lying on a tarp like the one that the police had described.

After more officers arrived at the scene, they were able to get close enough to Mr. Clark to disarm and arrest him.

Gruesome pictures posted to Mr. Clark’s Instagram account on Sunday showed Ms. Devins’s corpse and the caption, “I’m sorry Bianca.” In the time leading up to the killing, Mr. Clark had also posted a photo from inside a car with the caption, “Here comes Hell. It’s redemption, right?”

After her death, Mr. Clark posted photos of Ms. Devins’s bloodied body on a server on the chat platform Discord that she was known to frequent, the police said. Screenshots show a person posting photos of the killing and claiming to have committed it, then taunting Ms. Devins’s followers.

Lieutenant Coromato said that the police were looking at the posts, as well as similar ones on the anonymous message board 4chan, as part of their investigation.