At least 100 state and federal law enforcement officers are scouring an Ohio city, searching for a fugitive who has eluded them across multiple states for months after threatening to “put a bullet” in President Trump’s head, the authorities said.

The search became increasingly intense and focused on Sunday, after the authorities found a truck that they believe the fugitive, Shawn Richard Christy, 26, of McAdoo, Pa., had stolen. It was found in Mansfield, Ohio, a city of about 46,000, Robert Clark, a supervisory deputy United States marshal and one of the lead investigators, said in an interview on Tuesday.

On June 19, a federal warrant was issued for Mr. Christy’s arrest after he wrote a post on Facebook that threatened John M. Morganelli, the district attorney of Northampton County, Pa., and President Trump.

“Keep it up Morganelli, I promise I’ll put a bullet in your head as soon as I put one in the head of President Donald J. Trump,” he wrote, according to the authorities.