Who is suspected cop killer Eric Frein?

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As far as manhunts go, Eric Matthew Frein is an especially challenging target.

The suspected cop killer has extensive training as a marksman. Police say he's a survivalist who knows his way around the woods where they think he's hiding. And authorities worry he may be on the hunt for more officers.

Here's what we know about Frein, the 31-year-old wanted for the death of Pennsylvania State Police Cpl. Bryon Dickson and the shooting of another officer a week and a half ago.

Police say he'd been plotting his attack for months

Before Dickson and Trooper Alex Douglass were shot outside the state police barracks in Blooming Grove on September 12, Frein had been mulling an ambush for months, authorities say.

"Based on our investigations, we know Frein has prepared and planned extensively for months or maybe years," State Police Lt. Col. George Bivens said Sunday.

Investigators know Frein "has made statements about wanting to kill law enforcement officers and also to commit mass acts of murder," State Police Commissioner Frank Noonan said last week.

"He has very strong feelings about law enforcement and seems to be very angry with a lot of things that go on in our society."

Bivens said while Frein planned his attack and retreat, "We believe we are closing in on him."

He's the subject of an intense manhunt

Up to 400 law enforcement officers are searching for Frein, including members of the FBI, the U.S. Marshals Service and the Bureau of Alcohol, Tobacco, Firearms and Explosives.

They've zeroed in on the woods not far from Frein's family home in Canadensis -- about 20 miles from where Dickson and Douglass were shot.

His evasiveness has led schools to cancel classes and prompted authorities to ask hunters to avoid the woods in the area.

Although Frein is on the run and possibly armed, there is no indication that he is a danger to anyone other than law enforcement, Biven said.

"I am convinced that Frein has engaged in a personal battle with law enforcement, specifically the Pennsylvania State Police, and will likely stay focused on that fight," he said.

He's fascinated by the Cold War

Frein claims to have fought with Serbians in Africa and has studied Russian and Serbian languages, according to the FBI.

He even belonged to a simulation group that would stage re-enactments, Bivens said.

The simulation group, which police declined to name, would use non-lethal Airsoft guns.

But authorities believe Frein has turned a non-lethal activity lethal.

"It is believed that Frein has assumed the simulation role, and is now acting it out in real life," Bivens said.

Frein started wearing a "mohawk"-type haircut -- different from what he had worn for years -- apparently as part of his mental preparation for his attacks on the troopers, Bivens said.

His friend says he's a 'rational guy'

While authorities believe Frein has an intense grudge against law enforcement, one friend said his angst is actually against the federal government.

"He definitely let his opinions about the government be known," the friend, who wanted to be identified only as "Jack," told CNN.

"He was obviously a big critic of the federal government, but he never specifically targeted police when he was talking to me. No indications of really any malice towards law enforcement in particular. Most of his aggression was (toward) the federal government."

He's a skilled marksman

After a trail of clues -- including an abandoned Jeep registered to Frein's family -- led authorities to interview Frein's parents, Frein's father told authorities that two firearms were missing from the family's house.

One of those missing firearms was an AK-47, according to a probable cause affidavit.

The father said his son grew up with guns and was a member of his high school's rifle club.

When Frein shoots, his father told authorities, he "doesn't miss."