Chris Harper Mercer, who is accused of killing nine people at Umpqua Community College in Oregon, left behind a manifesto detailing his grievances.
The gunman opened fire on October 1 inside a classroom at Umpqua Community College and then killed himself when police arrived.
A police source told the Associated Press that Chris Harper Mercer complained in the document about not having a girlfriend.
The gunman also wrote that everyone else was “crazy” and that he was the sane one.
Chris Harper Mercer, 26, 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 realize I had so much going for me”, People magazine reported, quoting a source close to the investigation.
Meanwhile President Barack Obama announced on October 5 that he would travel to Roseburg, Oregon, on October 9 to visit survivors and relatives of the victims.
Chris Harper Mercer lived with his mother, Laurel Margaret Harper, in an apartment a few miles from the college.
Police have interviewed Laurel Margaret Harper and she told them that her son had been suffering from mental problems.
Laurel Margaret 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.
The mother wrote that Chris Harper Mercer used to bang his head against the wall and that she also attempted to counsel other parents whose children had similar difficulties, the New York Times reported.
Laurel Margaret 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 Chris Harper Mercer or a relative.
The gunman had six guns with him when he began his attack and the rest were found at his home, the newspaper reported.
Chris Harper 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.