Story highlights Discovery of two pipe bomb confirms Eric Matthew Frein's presence in wooded area, investigator says

Suspect may have been spotted within the past 24 hours

He is being sought in the death of Cpl. Byron Dickson in a September 12 ambush

Investigators found two fully functional pipe bombs and believe that a three-week manhunt may be taking a toll on suspected cop killer Eric Matthew Frein , Pennsylvania State Police Lt. Col. George Bivens said Tuesday.

"Over the last few days we have found several items that undoubtedly confirm Eric Frein is hiding in our search area," Bivens told reporters. "We have found a number of items belonging to Frein, some of which were hastily discarded."

Those items include a pair of pipe bombs found in the last two days that could be detonated via trip wire or fuse -- consistent with what Bivens called "Frein's non-confrontational and gutless efforts to kill or injure law enforcement from a distance." The bombs were capable of causing significant damage, he said.

"I can't get into his head but I'm sure that he is stressed at this point," Bivens said. "It's been a couple of weeks and I am certain that he is not sleeping well at night."

Frein may have been spotted within the past 24 hours in the rugged area where he has been sighted numerous times, Bivens told reporters. The suspect was wearing dark clothing and was seen between 75 and 100 yards away.

Police are looking for Eric Matthew Frein, suspected in September 12, 2014 ambush in Pennsylvania that left one state trooper dead and another injured.

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"We're confident that he's in the area," Bivens said.

Frein is sought in the death of Cpl. Bryon Dickson in a September 12 ambush outside the state police barracks in Blooming Grove. Trooper Alex T. Douglass was wounded in the attack.

Authorities have located several structures where they believe the suspect has stayed. Some items were found in a location that Frein may have used as an "overnight accommodation," Bivens said.

'We are not going anywhere'

As he has in the past, Bivens, who is spearheading the manhunt, had a personal message for Frein: "I am calling on you, Eric, to surrender. You are clearly stressed. You are making significant mistakes. We continue to take your supplies and your weapon stockpiles. While you are no doubt weakening our troopers' resolve is very strong. We are not going anywhere."

Bivens said Frein may have placed other explosive devices in the area and warned people to be very careful, especially since archery hunting season is about to begin.

The self-styled survivalist may have planned the ambush and retreat for months or years, police have said. The search area is a wooded area around the border of Pike and Monroe counties.

The area is not far from Frein's family home in Canadensis, in the Pocono Mountains.

The suspect has the advantage of knowing the terrain where he is hidden, officials said.

Poring through a variety of clues

Last week, police said a review of a computer hard drive used by Frein showed he planned the attack and his retreat into the woods for a couple of years.

The hard drive which Frein had access to indicated extensive Internet research on topics such as "how to avoid police manhunts, use of various law enforcement technologies and skills related to survival," Bivens said.

Police also were able to track Frein through his cell phone when he called his parents one time earlier this month, a federal law enforcement official told CNN. Frein hung up after one ring, the source said.

The search has yielded an empty pack of Serbian cigarettes, Bivens said. Frein claims to have fought with Serbians in Africa and has studied Russian and Serbian languages, according to the FBI, which last week named him one of its Ten Most Wanted fugitives. Soiled adult diapers were also found, perhaps used by Frein to stay in a stationary position for long periods of time.

Authorities have said that Frein hated law enforcement and that they believe that he's solely focused on hurting more officers but not civilians.

"I do think that we have significantly degraded his ability ... to adapt," Bivens said Tuesday.