People are calling them "ghost guns."
The guns are impossible, but these days they're showing up at more crime scenes.
A new intelligence report coming from Texas is warning law enforcement agencies across the country about these untraceable guns.
The parts are all legal to buy in the United States, but some are being assembled by people connected to criminal organizations, and now law enforcement experts are worried about terrorism.
"If you don't have a serial number, then you don't know where the gun came from," said Dr. James Ness, Founder of Ness Associates.
No serial number makes it harder for police trying to clean up the streets.
The report says more criminals are buying "80 percent receivers," which is the unfinished part of a semi-automatic rifle. A chunk of metal needs to be removed for the magazines and the trigger mechanism in order for it to be complete.
"There is no pocket cut to this area right here makes it an 80 percent,” said Rob Dunaway, President of American Spirit Arms.
No serial number is required because the ATF doesn’t consider the unfinished part a firearm, which means anyone can buy them without a background check.
Dunaway says most of the customers who buy the incomplete receivers are people who like to personalize their semi-automatic rifle.
"Some people buy them to store them for potential future use," Dunaway said.
Those folks are more worried about changes to the gun laws. But the report says criminal organizations across the world are also interested.
"Last year we got a call from San Felipe, just over the border from Yuma, from someone who wanted to buy five 80 percent lowers (receivers). That seemed strange. We didn't fulfill the order and we reported it to ATF," Dunaway said.
Between 2009 to 2011, ATF seized 191 80 percent receivers in Tucson that were headed to Mexico to be assembled.
But it's not just cartel groups to worry about. Law enforcement experts say organized militias should also be a concern.
"They are the ones most likely to do those things in my opinion. They are advocates of the Second Amendment and anti-government," Ness said.
Lone wolves with ties to international terror groups could also but parts and assemble their own guns under the radar to be used in an attack and no one would know.
Semi-automatic weapons are used in less than one percent of crimes in the U.S. Most criminals use handguns.
Also, most guns used in crimes are stolen.
Criminals looking to buy a weapon can get them from private sales without a background check. Sometimes, they will even sand down the serial number to attempt break ties with the gun, but forensics technicians can figure out that serial number.
Manufacturers say it takes hours to make rifles from scratch so it’s not the most efficient way used by most criminals.