Chris Harper Mercer opened fire on Thursday inside a classroom at Umpqua Community College and then killed himself when police arrived.
A police source told the Associated Press that Mercer complained in the document about not having a girlfriend.
He also wrote that everyone else was “crazy” and that he was the sane one.
Mercer is also said to have written that other mass killers “were denied everything they deserved and wanted” before going on to say he did not believe anything could “make me realise I had so much going for me”, , quoting a source close to the investigation.
Meanwhile President Barack Obama announced on Monday that he would travel to Roseburg, Oregon, on Friday to visit survivors and relatives of the victims.
The 26-year-old Mercer lived with his mother, Laurel Margaret Harper, in an apartment a few miles from the college.
Police have interviewed Ms Harper and she told them that Mercer had been suffering from mental problems.
Ms Harper, a nurse, posted on websites over past several years about the difficulties of having a son with Asperger’s syndrome, a form of autism that she also suffered from.
Ms Harper also wrote about her son’s knowledge of firearms, saying he was her main source of information about guns and revealing they kept Kalashnikov and Armalite semi-automatic rifles and a Glock handgun in the house.
Police recovered a total of 14 firearms and spare ammunition magazines that had been bought legally by Mercer or a relative. Mercer had six guns with him when he began his attack and the rest were found at his home, the newspaper reported.
Mercer had been discharged from the US Army in 2008 after failing to complete basic training. According to online postings, he had been in search of a girlfriend.
In a dating profile apparently belonging to Mercer on the Spiritual Passions website under the username Ironcross45, the non-smoking teetotaller described himself as “shy at first, but [I] warm up quickly, better in small groups”.
Survivors had previously suggested that Mercer had left a message for police.
Summer Smith, the mother of one of the survivors, said Mercer singled out her son, Matthew, and decided not to kill him.
“The shooter asked him to give the police something, and that if he did, he would live,” Ms Smith told CNN on Monday.
Ms Smith said Matthew was handed an envelope, with what appeared to be a thumb-size computer hard drive inside.
He then was forced to watch as Mercer shot and killed his classmates, Ms Smith said.
“Matthew said that he froze. He didn’t make a single move. He was afraid to look away,” she told CNN.