The British-born gunman who murdered nine people at an Oregon community college before killing himself ranted about not having a girlfriend and thinking everyone else was "crazy", a source close to the investigation has said.

The law enforcement source also said the mother of Christopher Harper-Mercer, 26, had told investigators he was struggling with some mental health issues.

Harper-Mercer complained in a manifesto he left behind about not having a girlfriend and seemed to feel like he was very rational while others around him were not, the official said.

In the manifesto, a couple of pages long, he wrote something to the effect of: "Other people think I'm crazy, but I'm not. I'm the sane one," the official said.

Meanwhile some staff and students returned to the Umpqua Community College campus in Roseburg for the first time since the shooting last Thursday and US president Barack Obama announced he would travel to Oregon on Friday to visit meet victims' families as he opens a four-day trip to the West Coast.

Mr Obama has renewed his call for stricter gun laws following the shooting and has expressed exasperation at the frequency of mass shootings in the US.

Classes do not resume until next week, but some students came to the campus to pick up belongings they left behind when they fled. Others met professional groups to discuss their trauma and grief.

A memorial was growing on the driveway leading to Snyder Hall, where Harper-Mercer opened fire. Besides those who died, nine people were wounded.

He apparently demanded to know his victims' religious beliefs before opening fire on the campus and killed himself after a shoot-out with police.

Carmen Nesnick, Harper-Mercer's stepsister, said he was born in the UK and travelled to the US as a young boy.

A group of eight held hands and bowed their heads in prayer in front of the building. Elsewhere, clusters of people chatted at picnic tables or near buildings.

In a courtyard near the centre of the campus, a therapy dog sat on a blanket with its handler. A woman, crouched down, wiped away a tear.

At least one student injured in the shooting was among those who returned, college president Rita Cavin said.

Reporters were barred from campus but taken on a brief tour. School officials designated an outdoor amphitheatre as a makeshift memorial, open only to staff and students for now. Flowers and balloons were positioned on tables, and markers were available for people to write messages on a banner that said "UCC Strong".

Chaplains who had been on campus said they were both helping with and participating in the healing process.