A law enforcement source confirmed for CBS News that a handwritten letter believed to have been written by suspect Eric Frein detailed the Sept. 12 ambush of two Pennsylvania state troopers.
CBS News senior investigative correspondent Pat Milton reports the letters discusses how he shot the troopers -- one of whom died from his wounds -- and then escaped. It was found by investigators during their manhunt.
The law enforcement source said that based on the details contained in the letter authorities believe it was written by suspect Frein.
On Sunday, officials revealed a text message sent by Frein shortly before the attack.
Frein's longtime friend received a text from the suspect around 6:15 p.m. on Sept. 12 which read "all is good." It was sent less than five hours before he allegedly opened fire.
The manhunt has now entered its fourth week. Authorities said Friday they have found caches of supplies in the rugged, heavily wooded search area. Tuna fish, ramen noodles and other food was recovered along with clothing and 90 rounds of ammunition from a campsite where police previously said they found two pipe bombs, according to investigators.
Court documents said Frein's wrote in his text message that he had stayed at the friend's apartment in East Stroudsburg the previous night. The friend was in Virginia at the time of the shooting, the affidavit said.
Frein's text also said he was "heading back to Delaware," and that he would return the following week, the affidavit said. Police have said Frein lied to relatives and other people that he was working in Delaware so he could prepare for the ambush.
Police obtained a search warrant for the apartment and recovered several boxes of ammunition and 10 dirty gun-cleaning patches, according to a document listing the items seized by police.
The friend is not charged in connection with the ambush and police have said Frein acted alone.