California shooters had semi-automatic weapons. What about other mass shootings?

The suspected gunmen in two separate mass shootings in California this week both had semi-automatic firearms – weapons that gun violence experts warn mass shooters are increasingly using in deadly public attacks.

Semi-automatic weapons automatically reload but require a shooter to pull a trigger to fire a shot.

• Twenty-two of the 66 mass shooters who killed four or more people in a public place from 2012 through 2022 used AR-15-style semi-automatic rifles, according to James Densley, co-founder of The Violence Project, a nonprofit research center.

• That's a significant increase. In the 123 mass shootings between 1966 and 2022, just five shooters used semi-automatic rifles, Densley said.

• Most guns used in mass shootings were legally obtained, according to the Violence Project’s database.

Monday, several senators reintroduced a federal "assault weapons" ban and legislation that would raise the minimum purchase age from 18 to 21. The ban on "assault weapons" – a broad term – would ban the sale, transfer, manufacture and importation of multiple kinds of semi-automatic firearms, among others.

The House passed a similar ban this summer, and President Joe Biden on Tuesday urged Congress to pass a federal ban. California is one of nine states with an assault weapons ban, which restricts specific models, including some semi-automatic weapons.

"The problem is that the United States is a confusing patchwork of gun laws and each state has its own set of rules, despite the fact that guns are durable goods and they can cross state lines," Densley said. "We need a universal standard."

Here's a breakdown of the weapons used in recent high-profile mass shootings.

Half Moon Bay, California

The 67-year-old man suspected of killing seven people in shootings on Jan. 23 near the Northern California community of Half Moon Bay was found with a semi-automatic handgun in his vehicle, police said. Authorities were still investigating where the weapon came from and whether it was purchased legally.

Monterey Park, California

The 72-year-old man suspected of killing 11 people in the Southern California city of Monterey Park on Jan. 21 used a "magazine-fed semi-automatic assault pistol," authorities said.

• Los Angeles County Sheriff Robert Luna said officers found 42 shell casings and a large-capacity magazine at the scene of the shooting, as well as a Norinco 7.62x25 handgun, registered to the suspect, in the suspect's van.

• The firearm wrestled away from the suspect at a second scene was a 9mm caliber semi-automatic MAC-10 assault weapon, and there were "some modifications" made, Luna said.

• Authorities also found weapons at the suspect's home, including a .308 caliber rifle, items that lead authorities to believe the suspect "was manufacturing homemade firearm suppressors" and at least "hundreds" of rounds of .308 caliber and 9mm caliber ammunition, he said.

• Authorities were still investigating where the weapons came from. Luna initially said he believed the firearm used in the shooting was illegal in California. He noted California "has some of the strictest gun laws in the country" but that legislation has changed over time.

Chesapeake, Virginia

A Walmart store manager opened fire before an employee meeting on Nov. 22, killing six people. The 31-year-old gunman legally purchased the 9mm handgun used in the shooting from a local store hours earlier, police said.

Colorado Springs, Colorado

A 22-year-old suspect fatally shot five people and injured 17 others at an LGBTQ bar and nightclub on Nov. 19. The suspect used two weapons: an AR-15-style semi-automatic weapon and a handgun – both "ghost guns" that lack serial numbers, according to The Violence Project.

Charlottesville, Virginia

A 22-year-old former University of Virginia football player fatally shot three football players and injured two others in a Nov. 13 shooting rampage on a charter bus. Authorities did not initially say what kind of weapon was used in the shooting but said a handgun was located nearby.

Raleigh, North Carolina

A 15-year-old suspect fatally shot five people, including a police officer and a 16-year-old boy, on Oct. 13. The suspect used a shotgun and a handgun, according to The Violence Project. It was not immediately clear how the teen obtained the firearms.

Memphis, Tennesee

A 19-year-old man livestreamed an hourslong rampage across the city that left three people dead and three wounded on Sept. 7. A video clip from the shooter's livestream of the killings showed him holding what looked to be a pistol.

Highland Park, Illinois

A 21-year-old suspect fired down from a rooftop on a Fourth of July parade, killing seven people and wounding dozens more. He legally bought guns in 2020 and 2021, including the semi-automatic rifle used in the attack.

Tulsa, Oklahoma

A 45-year-old man fatally shot four people at the St. Francis Health System campus on June 1. He legally purchased an AR-15-style rifle hours before the attack.

Uvalde, Texas

A former student fatally shot 19 students and two teachers and wounded more than a dozen other people at Robb Elementary School on May 24. He legally bought two AR-style rifles and ammunition shortly after his 18th birthday and days before the attack.

Buffalo, New York

A white man fatally shot 10 people and injured three others at a Tops supermarket in a predominantly Black neighborhood on May 14. He legally bought a Bushmaster XM-15 semi-automatic rifle in upstate New York two months before the shooting.

Dig deeper

• Nine states ban assault weapons. Does yours?

• Mass killing database: Revealing trends, details of every US event since 2006

This article originally appeared on USA TODAY: Monterey Park, Half Moon Bay shooters used semi-automatic weapons