When police arrived to investigate reports of a stabbing at a Franconia meat-packing plant in February 2015, they simply followed the blood. A crimson trail ran through the plant, out a door and across the snow into a shed.

"The trail was pretty extensive," Montgomery County Detective Ed Schikel said.

Schikel was among the first witnesses called as the trial opened Tuesday in Norristown for the man they found in that shed, Peter Jok Atem.

Prosecutors say Atem, 34, stabbed a co-worker, Danny Vasquez, 25, to death with a four-and-a-half inch butterfly knife in February 2015. He is charged with first-degree murder.

Atem's lawyer, Benjamin Cooper, said he would not contest whether his client killed Vasquez. But the killing was not premeditated, he said in opening arguments, and does not amount to first-degree murder.

Atem has post-traumatic stress disorder because he is a refugee from Sudan, Cooper said. He came to the United States in 2001 and was placed in foster care, but his lawyer said he has not recovered from his childhood trauma. He said Atem mistakenly believed he was acting out of self-defense when he killed Vasquez.

"He wasn't capable of forming a specific intent to kill," Cooper said.

Assistant District Attorney Alec O'Neill said the attack was planned.

Atem walked into a room at the plant the morning of the killing and attacked Vasquez, seated in a chair, said James Artis, a former co-worker of both men at the JBS Mopac plant.

Artis testified that he was making small talk with Vasquez when Atem silently stood in the doorway. He said Atem then jumped on Vasquez, holding his chest, but Vasquez was able to wrestle him off.

"We have fun around there, that's how we get through the day, I thought they were just playing," Artis said in his testimony.

He said both him and Vasquez tried to sort out the issue, but Atem never responded.

"I said, "Pete you alright today, you good?" Artis said. "Pete didn't say nothing."

Artis returned to work, but minutes later he said he heard screaming and saw Atem stabbing Vasquez in the chest. He said he didn't see Vasquez with any weapons.

Afterward, Artis said, he helped Vasquez to the plant's office to seek help.

Montgomery County Detective John Wittenberger said he spoke with Atem in the hospital the day after the attack, where he had been treated for his self-inflicted injuries. He said Atem admitted to stabbing Vasquez.

"He was messing with me for years," the transcript of the interview read. Atem said in the conversation he didn't routinely carry a knife with him, but he did that day.

"I was angry with him, but I didn't want to kill him," the interview transcript said.

A copy of the handwritten note that Atem left beside him after he stabbed himself, ended with, "See you in hell. Life for life."

The trial, before Montgomery County Judge Gail A. Weilheimer, will resume Wednesday morning.