State Police Lt. Col. George Bivens said that the explosives could have been detonated by a tripwire or lighted fuse.
He added that this was consistent with the crazy survivalist's "gutless efforts to kill and injure law enforcement from a distance."
Officers searching for Frein had also found other supplies that the fugitive had "hastily" left behind but he did not say what they were.
I'm calling on you to surrender," Bivens said. "You are clearly stressed. You are making significant mistakes."