A US Marine Corps veteran with suspected mental health issues killed 12 people in a busy bar in California, including a policeman, officials say.

The shooting began at 23:20 local time on Wednesday in Thousand Oaks about 40 miles (65km) north-west of Los Angeles.

At least 200 people were reportedly inside the Borderline Bar and Grill, which was hosting a student line-dancing night.

Police named the suspect as 28-year-old Ian David Long.

Earlier this year, police mental health professionals cleared him after he was found behaving “irrationally” at his home, said authorities.