Authorities say a journal left by ambush suspect Eric Frein provides a “cold-blooded and absolutely chilling account” of the night he allegedly opened fire outside a Pennsylvania state police barracks, killing one trooper and injuring a second.

Frein wrote that he saw his shot and took it, watching from a distance as his victim – who turned out to be Cpl Bryon Dickson – dropped to the ground. “I was surprised at how quick,” he wrote, according to police.

Authorities found the multipage journal in a bag of trash at a wooded campsite they believe was used by the 31-year-old suspect. The document describes how Frein fled the shooting scene in a Jeep, but inadvertently ran into a retention pond and took off on foot – a botched getaway he termed a “disaster”, according to a police affidavit filed Wednesday.

Frein is charged with first-degree murder and other offenses in the 12 September ambush that killed Dickson and wounded trooper Alex Douglass. Hundreds of law enforcement officials have been looking for Frein – described by authorities as a survivalist and expert marksman with a grudge against law enforcement – in the woods around his parents’ home in Canadensis, in the Pocono Mountains.

A Jeep that ambush suspect Eric Frein is believed to have driven into a swampy area. Photograph: AP

The journal was found 29 September, when authorities discovered a hastily abandoned campsite at which they also found explosives, ammunition, food, clothing – and a water bottle whose DNA matched Frein’s, according to the affidavit.

Authorities are analyzing the document but are convinced it was written by him.

“I will tell you, after reading this cold-blooded and absolutely chilling account, I can only describe Eric Frein’s actions as pure evil,” said Lt Col George Bivens at a news conference Wednesday.

Bivens said the journal contains details that only would’ve been known by the shooter. Notably, police said, the author does not identify the troopers by name, suggesting they were targeted at random.

After describing how the first victim. Dickson, dropped to the ground after being shot, the journal’s author wrote: “I took a follow-up shot on his head/neck area. He was still and quiet after that. Another cop approached the one I just shot. As he went to kneel, I took a shot at him and [he] jumped in the door. His legs were visible and still,” according to the affidavit.

A campsite that officials say was used by suspected killer Eric Frein. Photograph: AP

The shooter then ran to his Jeep and sped off. He turned down an access road to avoid a roadblock and wound up in the retention pond, the journal said.

“Disaster”, Frein allegedly wrote.

Other items found by police, including Frein’s checkbook and cellphone, indicate he might have left the campsite in a hurry under pressure from the manhunt. Police say he used the phone to call his parents six days after the shooting, and they were able to trace the signal to a location about three miles from the home.

In addition to confirming the journal’s existence, authorities also elaborated on two pipe bombs they previously announced finding at the campsite, saying they were packed with metal shrapnel and explosive powder consistent with the kind of powder found in Frein’s bedroom.

Based on the discovery of the pipe bombs, police filed additional charges against Frein on Wednesday – two counts of possession of weapons of mass destruction.

Officers believe they have spotted Frein in the forest, but the distance and rugged terrain allowed him to slip away. The most recent sighting came Tuesday afternoon, when an officer thought he had a brief glimpse of Frein from about 200 yards away, Bivens said. That was the fourth sighting by police or the community since Friday, he said.