Another Multiple Shooting Incident in California

Once again, the question comes to my mind - where did these guns come from? This could only have been a more dangerous incident if other people had also opened fire in response.  The resulting cross fire would almost certainly have killed or injured even more victims.
From MSNBC.com:

3 dead, 3 wounded in California party shooting

updated 10/2/2011 11:04:13 PM ETSAN LEANDRO, Calif. — Three people were killed and three others wounded when at least two gunmen opened fire as partygoers left a Northern California warehouse early Sunday, authorities said. The gunmen started shooting as a group of people walked out of the building around 1:20 a.m., San Leandro police Sgt. Doug Calgano said.
Three people were pronounced dead at the scene. Police did not know the condition of the three who were wounded. The names of victims had not been released, but Roger Kiel told San Francisco television station ABC7 that his daughter, Shanice Kiel, 19, was one of the people fatally shot.
"I'm devastated" he told the station as police examined a bullet-riddled SUV in the background. "I'm looking at my car with the bullet holes in my windshield where I sit. So, I know my daughter suffered a horrible death."
At least 100 people attended the party, which had been advertised on the Internet, Calgano said. He was not aware of a permit being issued for the party, which would have been required for a gathering of that size.
As the shooting began, many of the partygoers fled.
"Obviously it was a chaotic scene," Calgano said.
The gunmen were armed with semi-automatic handguns, but police have not determined how many shots were fired. Investigators were trying to establish if the gunmen had attended the party, or arrived as people left the party. It was not clear if the gunmen were in a vehicle or on foot when they opened fire.
No arrests have been made, and police have not determined a motive.