Law enforcement shot and killed the 19-year-old suspect at the Gilroy Garlic Festival, but not before he opened fire on the crowd, murdering three people, including a 6-year-old boy. Investigators are attempting to figure out why Santino William Legan opened fire on the storied summer festival in central California, and whether he had any assistance:

Gilroy Police Chief Scot Smithee told reporters in a late-night briefing that Gilroy officers engaged a suspect within a minute of shots ringing out and killed him.

Smithee said witnesses told authorities at least one other person may have been involved in the shooting but there was no confirmation of that or the role he or she might have played. He said a manhunt was underway for the possible accomplice.

Federal law enforcement sources told CBS News the FBI was among numerous law enforcement agencies at the scene assisting Gilroy police, who were leading the investigation.

Smithee said the suspect or suspects appeared to have cut through a wire fence at a creek bordering the festival grounds to bypass heavy security to gain access.

There was no early word on a possible motive. Witnesses told CBS News the gunman appeared to be firing at random. Some witnesses said he suddenly appeared from behind a stage before beginning to shoot.

According to CBS’ San Francisco affiliate, the shooter came out from behind a stage to start his spree. In this video, one can see the band react to the shots being fired before the videographer runs for her life. The lead singer heard the shooter make a declaration of his motive after he opened fire:

Tinman lead singer Jack van Breen was stunned when he looked up and saw a man opening fire on the crowd with what looked like an assault weapon.

Van Breen, whose band was the last of the day and performing an encore at the annual Gilroy Garlic Festival, told KPIX 5 he heard someone shout, “Why are you doing this?” He said the person responded, “Because I’m really angry.”

Ugh. This sounds like a mental-health issue again, although it’s too early to fix on any one problem. Now that the shooter has been identified, we’ll know whether he was on law-enforcement radar before the incident soon enough. California has one of the strictest gun-control regimes in the country, but of course that issue will come up again, likely well ahead of the evidence curve, if history is any guide.

Police say they are looking for an accomplice, but that’s a common thread in these incidents as well. Usually — not always, but usually — the shooters act on their own, and second suspects turn out to be the product of very understandable witness confusion after a traumatic incident. The police will run down the leads professionally, but it would be surprising if an accomplice turns up. These kinds of malcontents aren’t usually social in nature.

We’ll keep up with this story as developments warrant.