Officials have found the AK-47 style assault rifle Eric Frein, 31, may have used during the ambush that left a state trooper and ex-U.S. marine dead, police said Sunday night. Several magazines of ammunition were also found, which led police to believe Frein had planned the assault far in advance.

"Based on our investigations we know that Frein has planned and prepared extensively for months and maybe years," Pennsylvania State Police Lt. Col. George Bivens said at a news conference Sunday.

The weapons were found in a densely wooded area in Canadensis, Pennsylvania, which is where the ambush that killed Cpl. Bryon Dickson and critically wounded State Trooper Alex Douglass Sept. 12 took place. The amount of undeveloped land surrounding the shooting area has most likely hindered law enforcement’s ability to find Frein.

"Where the shooting took place there's tens of thousands of undeveloped acres of woods," Dave Price, who lives nearby and offered his land as a law enforcement staging area, told NBC News.

Police said Sunday they had surrounded the area where Frein was believed to have gone after the shooting and encircled the land around his parent’s home in Canadensis. 

The now week-long manhunt involves up to 400 law enforcement officers, including members of the Federal Bureau of Investigation, the U.S. Marshals Service and the Bureau of Alcohol, Tobacco, Firearms and Explosives, according to CNN. Frein has been put on the FBI’s most wanted list.  The FBI described him as “a heavy smoker, a weapons enthusiast, and a survivalist.” He's considered armed and “extremely dangerous.”

Schools in the area have canceled classes and residents have been urged to stay inside when possible, but police said it’s unlikely that Frein will target civilians.

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.