Police say the assault weapon that was used in the attack is uncommon and was one of several high-power guns in Huu Can Tran's possession that night.
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In the video, Tran is seen carrying an assault pistol with an extended magazine.
"The firearm which was wrestled away from the suspect at the Alhambra scene was a 9mm caliber semi-automatic MAC-10 assault weapon," said Los Angeles County Sheriff Robert Luna.
David Pucino, deputy chief counsel for the Giffords Law Center to Prevent Gun Violence, said it is illegal to buy or sell a MAC-10 in California.
"The handgun that he did use in the shooting that had that extended magazine, I will note, is not common. It's a very uncommon weapon," Pucino said.
He added investigators will have to figure out when Tran got a hold of the weapon.
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"I will note that there are legacy provisions under California law that allow people who possess that weapon before they were initially banned to continue to possess them so it's possible that the individual had the gun in his possession for that length of time," Pucino said.
Luna detailed some of the items recovered at Tran's home in Hemet during a search warrant. They included a .308 caliber rifle, hundreds of rounds of ammunition and numerous electronic devices like cell phones and computers.
There were also items that led investigators to believe Tran was manufacturing homemade firearm suppressors.
"Those are almost certainly illegal. In the United States, suppressors are regulated under a law called the National Firearms Act, which is the law that regulates machine guns going back to the 1930s, and as a general matter, you cannot just go and manufacture those in your homes," Pucino said.
He said suppressors, also known as silencers, are added to the front of guns to lower its volume when fired.
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"The suppressor makes it more difficult to identify by audio hearing where that shooting is happening from," Pucino said.
Pucino said even though California has the strictest gun laws in the country, many guns come from out of state.
"A lot of the crime guns that are used in California might be banned in California but they're obtained out of state and brought in," he said. "That's because our gun laws stop at state lines but guns don't."
Luna said all the weapons recovered so far need forensic examination and additional investigations need to be conducted to determine where the suspect got the guns.