ROSEBURG, Ore. – No criminal charges will be filed in the 2015 mass shooting at Umpqua Community College, authorities announced Friday.

The investigation was officially completed nearly two years after the shooting on Oct. 1, 2015.

It was the deadliest shooting in Oregon history.

“All investigative indications are that the shooter acted alone in this incident,” the Douglas County Sheriff’s Office said in a press release.

Read more: UCC victim shielded fellow student before he died

Christopher Harper-Mercer killed nine people and wounded nine more at the UCC campus before shooting and killing himself during a confrontation with two Oregon State Police troopers.

Federal investigators said the six guns Harper-Mercer carried to campus and the seven guns found in his home were all legally purchased.

Investigators conducted hundreds of hours of interviews as part of the investigation.

Authorities release shooter's 'manifesto'

The detailed report on the shooting investigation includes a six-page, typewritten "manifesto" that says Harper-Mercer had studied the methods of other mass killers.

In the document left on a thumb drive for police to find, Harper-Mercer says he serves the "demonic Hierarchy" and will become a demon when he dies and return "to kill again and again."

He also makes it clear that he idolizes other mass shooters and says he has studied their methods to figure out how to kill more people.

More: UCC shooting hero hopes for 'a community healing'