LOS ANGELES -- A handgun that looks and fires just like a Glock 9mm has no serial number, is completely untraceable and 100 percent legal. It's known as a "ghost gun," and we bought one online with no background check or waiting period.

It's not technically a firearm, because the part that would be registered -- the lower receiver -- still needs work. But a do-it-yourself kit came with all the necessary parts, even the drill bits and a plastic template showing exactly where to drill the holes.

"It's not going to take a tremendous amount of gunsmithing skills," said Scott Reitz, a retired Los Angeles Police Department SWAT officer.

CBS News got a ghost gun online and assembled it within a few hours CBS News

Reitz agreed to supervise while we built the gun. Following an instructional YouTube video, it took less than three hours to build. After a safety check and test-firing, Reitz put the gun through its paces.

Countless websites offer so-called ghost gun kits for everything from handguns to AR-15s and AK-47s.

"They're trying to appeal to a certain segment of the population," said Dave Hamilton, senior special agent for the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). "Felons who can't go to a gun store and legally purchase a firearm, or people who just don't want the government knowing what type of firearms they have."

Hamilton showed us several different ghost guns. Many had come off the streets. They recently confiscated 46 in a raid at a single home.

Ghost guns have to be assembled CBS News

"There's nothing the ATF can do," said Ginger Colbrun, with the ATF's Southern California Public Information Office. "These firearms are with gang members, these firearms are being are being found at various crime scenes all over the country."

Police say unregistered, homemade guns were used last November when Kevin Neal killed five people in a Northern California shooting spree, and in 2013, when John Zawahri shot and killed five in Santa Monica

"ATF can't go shut down the people who are selling these parts because these parts are not regulated," said Colbrun.

There's no limit on the number of ghost guns one person can own.

"It's really up to those companies to be responsible," Colbrun said. "They're the ones that are going to have to live with themselves."

A 50-year-old gun law makes homemade guns legal to own, and the only way to regulate the firearms is for Congress to take legislative action.