Last Updated Sep 30, 2014 4:28 PM EDT
Investigators searching the dense woods of Pennsylvania's Pocono Mountains for the suspect in the deadly ambush of state troopers have found two "fully functional" pipe bombs that are believed to have been left behind by the suspect, officials said Tuesday.
Lt. Col. George Bivens said that investigators have found "several items" attributed to Eric Frein, including the pipe bombs which could be detonated by trip wires or a lighted fuse.
The pipe bombs had the capability of causing substantial damage, Bivens said.
The search for Frein entered its 18th day Tuesday. He is charged with opening fire at the Blooming Grove barracks during a shift change on the night of Sept. 12, killing Cpl. Bryon Dickson and seriously injuring Trooper Alex Douglass.
Bivens said he was confident that the manhunt has "significantly degraded" Frein's abilities.
"I'm certain he's not sleeping well at night," Bivens said.
Investigators searching for Frein have previously found an AK-47 and ammunition in the woods.
Frein, 31, is described by authorities as a survivalist, marksman and war re-enactment enthusiast who planned the attack for years, extensively researching how to avoid police manhunts and experimenting with explosives. Frein has held anti-law enforcement views for many years, police said.
Authorities believe they have Frein contained within a 5-square-mile perimeter around his parents' home.
Bivens said Tuesday that he doesn't believe this was Frein's original getaway plan. Frein's car was found submerged in a swamp near the barracks a few days after the shooting.
"I think things went wrong with his plan," Bivens said.
Bivens called on Frein to surrender.
"You are clearly stressed," he said. "You are making significant mistakes."
Two schools closed for more than a week due to the massive police manhunt have reopened in northeastern Pennsylvania.
Some districts canceled classes during the early days of the manhunt because officials didn't want buses traveling through the search area to pick up children. The Pocono Mountain School District resumed regular bus service Monday, with the exception of a few stops.
CBS Philly reports that there are concerns that the manhunt could adversely affect the busy fall tourism season in that area.