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What to know about 'ghost guns' — the deadly weapons that can't be traced

A recent school shooting in Maryland has put the spotlight on the proliferation of untraceable "ghost guns" that can be assembled at home.

A growing number of police departments across the country say the proliferation of homemade firearms or "ghost guns" is becoming a serious issue, with the latest example coming in a shooting at a Maryland high school last week.

An investigation by NBC News also found that instructions for making the unregistered weapons at home could be easily found on websites like YouTube.

"These operate, function, look, feel, shoot just like regular gun because that’s what it is, but it’s not serialized," San Diego Police Department Lt. Paul Phillips told NBC News correspondent Erin McLaughlin on TODAY Monday.

Police in Montgomery County, Maryland, say that a ghost gun was used by a 17-year-old junior in a shooting at Magruder High School in Rockville on Jan. 21 that left a 15-year-old sophomore in serious condition.

The victim is expected to survive, and the shooter has been charged as an adult, Montgomery County Police said.

Police believe the shooter used a build-it-yourself firearm that has no serial number and is nearly impossible to trace.

Police departments nationwide are calling the guns a growing weapon of choice for criminals.

Three years after YouTube banned ghost gun tutorials, an NBC News investigation found dozens of videos still on the site with step-by-step directions for building the weapons.

YouTube told NBC News its guidelines prohibit that content, and it has since taken down the videos flagged by NBC News for violating its policy.

Police say ghost gun kits are readily available on mainstream platforms, and a legal loophole allows anyone to buy one without a background check because the kits only contain partially completed gun parts. The kit is not considered an illegal firearm until the gun is assembled.

The number of seized ghost guns seized nationwide has gone up almost five times between 2016 and 2020. The number has gone up more than tenfold since 2019 in San Diego, where police have established one of the first ghost gun units in the country.

"You can see the holes where the specific drill locations are," Phillips said. "And so when we come into a house and we see three or four of these sitting down in the kitchen, we know what’s going on. They’re building ghost guns."

McLaughlin sat down with two of the San Diego unit's undercover agents, whose names and likenesses have been concealed.

"If you can do IKEA furniture, you can do this," one detective said about the ease of assembling the ghost guns. "Honestly, if you have all the right parts and you have the right type of tool, you can do it in your car while you’re driving home from wherever you just bought the thing."

The other detective said a person could buy a kit and assemble a gun within an hour.

Bryan Muehlberger and his family have felt the devastating effects of ghost guns firsthand. His 15-year-old daughter, Gracie Anne Muehlberger, was shot and killed by a 16-year-old classmate using a ghost gun in a 2019 shooting at Saugus High School in Santa Clarita, California, police said.

Demonstrating the ease of obtaining such a weapon, Muehlberger told McLaughlin he personally was able to find and purchase a ghost gun kit on his cellphone within minutes by using his late daughter's name.

"It was really disheartening when a box showed up in my mailbox into my front door with my daughter’s name on it, who’d been deceased since November of the previous year," he said.

The San Diego detectives said technology is outpacing the law. Ten states currently have enacted laws to partially address the proliferation of ghost guns.

"The internet is so much bigger now than it was even five six years ago, and what you can get online, how it teaches you how to build these things," one of the detectives said.

The Justice Department has proposed a federal rule regulating the sale of homemade gun kits that would require buyers to pass background checks and force manufacturers to add serial numbers to the parts, but the rule is still pending.