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SAN DIEGO — Authorities say the man suspected of killing one person and wounding four others in a pair of shootings downtown late Thursday used a “ghost gun” in the attacks.

The term refers to firearms that lack serial numbers because they’ve been purchased in individual parts, often from kits sold online, and then assembled by the owner. This also allows people to buy and build the weapons without passing a background check.

In the case of Thursday’s shooting in the Gaslamp Quarter, it was an unregistered 9 mm handgun wielded by suspected gunman 32-year-old Travis Sarreshteh, according to police.

“We’ve seen a 169% increase in unserialized, or ‘ghost guns,’ from 2020, ” San Diego Police Chief David Nisleit told reporters Friday. “It’s very concerning. We’re seeing a lot of ghost guns. I think I reported not more than a month ago that about one in four of every gun we recover right now is a ghost gun.”

The largely untraceable weapons have taken a prominent place in recent calls for gun reform following mass shootings across the nation, and President Joe Biden took aim at the weapons in a recent series of executive orders on gun laws.

While the weapons have been used in some mass killings, including the Saugus High School shooting in Southern California, officials from the West Coast to New York say they more often encounter the guns at day-to-day crime scenes.

As part of Biden’s new orders, the Justice Department “will issue a proposed rule to help stop the proliferation of ‘ghost guns,'” the White House says. The rule is expected to effectively classify the kits used to build guns as firearms, making them subject to the same federal laws surrounding pre-assembled weapons, but specific details have not yet been released.

Beyond the federal regulations, California already has passed a law stating that starting in 2025, most individual gun parts will have to be sold through a licensed firearm dealer and will require a state background check. According to the Giffords Law Center, officials have agreed to speed up implementation of the policies, with most in place by 2022.

In San Diego, investigators were still trying to determine a motive for the shootings Friday. Sarrshteh lives downtown and has a criminal history but wasn’t generally known to officers, according to Nisleit. Detectives noted that he seemed to be looking for trouble as he walked through downtown late Thursday, “bumping into people” and “trying to pick a fight.”

In his comments Friday, San Diego’s police chief framed the shootings as part of a rising trend of violence in San Diego and other U.S. cities this year.

“Just a month ago, we released our 2020 annual crime statistics which showed an increase in violent crime consistent with other cities across this nation. Unfortunately, early statistics for 2021 are showing that we’ll see an increase again this year,” Nisleit said. “This is what we’ve seen so far. A 4% increase in calls from the community, a 50% increase in shooting investigations for the first two months of this year compared to 2020.”

“We’re seeing a prevalence of guns and folks in possession of firearms in our community and this is causing not only me as the chief of police but our police department great concern,” he continued. “I want all San Diego to know San Diego PD is working day in and day out to protect our city. We want our residents and our visitors to feel safe and we want to make certain we’re working collaboratively to get guns off the streets.”