Dean said in California, the gun is designed to hold 10 rounds, and then one in the chamber, but the weapon used in the shooting had "an extended magazine on it."
Authorities do not yet know how many rounds were fired. According to ABC News, the gun was purchased legally.
The shooting left 13 people dead, including the gunman, identified as Ian David Long. The preliminary information was that the suspect killed himself.
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Dean said detectives are investigating whether Long suffered from post-traumatic stress disorder. Long served in the U.S. Marine.
According to Dean, Long had minor run-ins with law enforcement, including an incident in April in which deputies were called to his home in Newbury Park on a report that he was acting erratically.
"They went to the house, they talked to him. He was somewhat irate, acting a little irrationally. They called out our crisis intervention team, our mental health specialist, who met with him, talked to him and cleared him, didn't feel that he was qualified to be taken under 5150," Dean said.
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