Falling leaves and cold, wet weather will make the search for the ambush killer of a state trooper easier, Lt. Col. George Bivens said Friday.

Bivens said police have been using thermal imaging to hunt through the rugged woods of Barrett and Price townships for Eric Frein, 31, an expert marksman and self-styled survivalist accused of shooting to death Cpl. Bryon Dickson and wounding Trooper Alex Douglass at the Blooming Grove barracks on Sept 12.Bivens said that thermal imaging technology works better when the canopy of leaves is gone. Further, he said, Frein will be seeking shelter and warmth, making him more vulnerable.He'll likely also be raiding dumpsters at businesses for food. Bivens asked business owners to carefully watch surveillance videos and security cameras.On Tuesday, police found what they say is a campsite hastily vacated by Frein, who left behind backpacks; bags; what appears to be a poncho; food, including cans of tuna and instant noodles; clothing; two pipe bombs and a "significant amount of ammunition," Bivens said.There were more than 90 rounds of .308 ammunition, he said. That's the type of ammunition police say he used in the shootings.They've also found an AK-47 style weapon and ammunition, discarded packs of the Serbian cigarettes Frein favors, and items they believe he had squirreled away for future use.Bivens said police are confident the items are Frein's. The FBI said Friday that diapers police believe were left by Frein in the search area and found last week could not be conclusively linked to the accused killer by DNA because they had been exposed to the elements."Because we continue to push Frein, and seize the items he needs to survive, we believe he will be forced to search for food and shelter in other locations," Bivens said.Police are looking through cabins, deer stands, and vacant homes in the search area."We remain confident Frein is in the search area, near the (Monroe/Pike counties) border," Bivens said.Police have had what they say are several sightings of Frein, the latest from 75 to 100 yards away earlier this week.Police between 3 p.m. Thursday and 3 p.m. Friday drained the pond, about two miles north of the Blooming Grove barracks, where Frein's Jeep was found on Sept. 15. No evidence was found in the pond, Bivens said.They've also had three unconfirmed sightings this week by residents and police, he said. Police have a "high level of confidence" that a sighting of Frein earlier this week was indeed him.Bivens said Frein is armed and dangerous, and advised people to immediately call police if they see anything suspicious. Bivens said police have been talking with Frein's family, and the possibility of them making a public plea for him to surrender has come up.With the search for Frein entering its fourth week, more than 1,000 police officers are focusing their search on a five-square-mile area in Barrett and Price townships and close to the Canadensis home where Frein lived with his parents. The search area is flexible, with officers investigating wherever new information leads them.The terrain is rugged and heavily wooded. Bivens said police were given ski poles and sticks to poke and prod the ground."You could literally walk right past someone in some of this terrain and not see them unless you stepped on them," Bivens said.The state Department of Conservation and Natural Resources has closed to hunters sections of the Delaware State Forest in which police are searching. The game commission on Wednesday closed its lands to hunting and trapping in seven townships: Barrett, Price, and Paradise townships in Monroe County, and Blooming Grove, Porter, Greene and Lehman townships in Pike County.Roads remain open.