DALLAS, Pa. — A man who police say trapped and killed three co-workers inside a closed northeast Pennsylvania grocery store overnight Thursday left an online trail behind that includes praise for the 1999 Columbine High School shooters and expressions of deep frustration about the world around him.

Wyoming County District Attorney Jeff Mitchell said a Twitter feed that includes a 42-minute film about a violent massacre, posted about the time of the killings, is believed to have belonged to 24-year-old Randy Stair of Dallas, Pennsylvania.

In that film, Stair praised Columbine shooters Eric Harris and Dylan Klebold as heroes and kissed and fondled a loaded shotgun.

Police say Stair brought two pistol-grip shotguns to work at the store in rural Tunkhannock, about 150 miles northwest of New York City (241 kilometers), blocked store exits and began shooting shortly before 1 a.m. A fourth co-worker eventually escaped unharmed and called police. Stair also killed himself.

“This is really a mental health situation that utterly spiraled out of control,” Mitchell said. “I think he had longstanding mental health issues that resulted in this horrible tragedy.”

State police said he spent the first 90 minutes of his shift blocking exits with pallets and other items. After the store closed Stair retrieved a duffel bag from his car with the two shotguns, and began his attack.

He killed Terry Sterling, 63, of South Montrose; Victoria Brong, 26, of Factoryville; and Brian Hayes, 47, of Springville.

“It’s just unspeakable, it really is,” Mitchell said. “These people went to work and they lost their lives because they went to work. It’s senseless.”

He said Stair apparently did not like one of the victims, the night manager. It wasn’t immediately clear which of the victims held that job.