"Have been fully briefed on the terrible shooting in California. Law Enforcement and First Responders, together with the FBI, are on scene. 13 people, at this time, have been reported dead," Trump said in a Twitter post.
The Ventura County Sheriff's department has said the 13 people include the gunman and the first sheriff's deputy who responded to the scene.
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Trump added in a second statement that the California Highway Patrol and police showed "great bravery" and responded to the scene of the shooting within 3 minutes.
He also said the sheriff's sergeant who was the first to arrive at the scene died in hospital.