Accused cop-killer Eric Frein used a computer to research how to avoid police manhunts, use of various law enforcement technologies and skills related to survival.

But Frein removed the hard drive from his own computer before the Sept. 12 ambush attacks on Cpl. Bryon Dickson and Trooper Alex Douglass at the Blooming Grove state police barracks in Pike County.

Dickson, 38, was killed; Douglass remains in critical condition but is recovering.

The evidence recovered from the hard drive supports the belief that Frein planned the attack for at least two years, Pennsylvania State Police Lt. Col George Bivens said at a press conference Friday.

The computer was not Frein's, but was not a public computer, Bivens said.

He said Frein had removed the hard drive from his own computer before the attacks.