SAN FRANCISCO, July 28 (Xinhua) -- Police said that there might be a second shooter involved in an incident late Sunday at the annual Gilroy Garlic Festival in Northern California, which killed four people and injured 15 others.

Gilroy Police Chief Scot Smithee believed that the first shooter cut through the fence to gain access to the festival, and the shooter was killed by the police minutes after the first shots rang out.

Video footage posted on social media showed that festival revelers were running in panic from the scene and seeking places to hide. At least one gunshot was heard in the video.

A witness said that the gunman was spraying bullets on people with a "semi-automatic rifle."

Five victims were treated at a hospital in Santa Clara Valley, including two people suffering major trauma, after being shipped from St. Louise Regional Hospital in Gilroy, the NBC Bay Area TV channel reported.

The Gilroy Police Department cautioned the public on its Twitter account that the situation at the shooting venue was still active and advised them to find their loved ones at a reunification center newly set up at nearby Gavilan College.

Sunday was the last day of the annual three-day festival, which has attracted thousands of garlic lovers.