He added that it is possible it was workplace-related or that there were mixed motives.
Barack Obama was speaking in the Oval Office to assure Americans that authorities will get to the bottom of what happened.
"He also called for people to wait for facts before making judgments."
US president said many Americans feel there is nothing they can do about mass violence. But he added: "We all have a part to play."
Obama said the nation must make it harder to carry out violence. But he acknowledged the threat cannot be eliminated completely.
He said it will be important for all Americans, including state legislators, to see what they can do. belfasttelegraph.co.uk reported.
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