Matthew Eric Frein, 31, of Canadensis, Pennsylvania, is shown in this undated handout photo provided by the Pennsylvania Department of Transport September 16, 2014. Frein, described as an anti-law enforcement survivalist, is the prime suspect in the ambush of two troopers outside their barracks last week. Frein is armed and extremely dangerous, according to State Police Commissioner Frank Noonan, adding he was believed to be on the loose with a .308-caliber rifle and scope. REUTERS

A 31-year-old survivalist being sought in connection with a double shooting in Pennsylvania is thought to be leaving a trail of used diapers and Serbian cigarettes for police in a deadly game of cat-and-mouse.

Eric Matthew Frein, who is suspected to have shot and killed US State Trooper Corporal Bryon Dickson, 38, and wounding Alex Douglass in an ambush outside a barracks in Blooming Grove, is thought to be hiding out in woodland in the Pocono Mountains.

Police hunting for Frein say they have found soiled diapers used by professional marksmen when forced to stay in one place for a long period of time. The diapers are being tested to see if they were used by Frein. Police claim to have seen him in the distance, but he still evaded capture.

Serbian cigarettes have also been found, which is seen as significant as Frein is said to speak Serbian and has claimed to have fought with Serbs in West Africa. Last week an AK-47 and ammunition were found, also probably belonging to Frein, but he is still believed to be armed and dangerous.

Frein is said to be a gun fanatic with a loathing of law enforcement agencies. He also has a fascination with Eastern European armies and briefly appeared in a movie, Lustig, playing a German soldier.

New footage has emerged of Frein in a documentary about a Vietnam War re-enactment in which he describes the difficulty of finding someone in the terrain of northeastern Pennsylvania where he is now believed to be hiding. He called his parents on 18 September enabling police to narrow down the area he might be hiding.

Police have stressed that they believe Frein only to be a danger to them and not to the general public. Schools in the area have now re-opened. However, many locals say they will remain nervous until Frein is captured or killed. A $100,000 (£61,550) reward has been offered for his capture.

State police spokesman George Bivens said Frein seemed to have been researching ways to evade capture for some time.

"We are convinced that Eric Frein has been planning this attack at least for a couple of years," said Bivens. "This information is supported by a search of a computer hard drive."

Frein is believed to have a Mohawk haircut and be wearing a brown-and-gold windbreaker, khaki shorts and trainers, carrying dark-green backpack with a black trim.