"Ghost guns" are displayed at the headquarters of the San Francisco Police Department in  2019.
CNN  — 

The Los Angeles County district attorney wants credit card companies to halt payments for online purchases of assemble-at-home kits for so-called ghost guns – untracked and untraceable firearms that are finding their way into communities across the country in increasing numbers.

District Attorney George Gascón challenged credit card companies on Tuesday to go beyond what law enforcement can do in limiting the proliferation of ghost guns.

“American Express, Mastercard and Visa have the ability to go beyond what any law enforcement agency, legislature or city council can accomplish,” Gascón said in a news release. “We are asking these companies to join us in stemming the flow of ghost guns into our communities by preventing a ghost gun kit from being sold with a few mere clicks on a smartphone or computer.”

The build-it-yourself guns are made from kits that can be sold to anyone. The kits cost $350 to $500, according to Gascón’s office. He says the guns are easy to purchase, especially to those who want to evade a background check including underage buyers, felons, or people with criminal pasts or mental illness.

Data shows that these guns have been involved in shootings and found at crime scenes with increasing frequency. Several cities have reported sharp increases in the number of ghost guns recovered over time – and while they make up a relatively small percentage of the total number of guns recovered by law enforcement, that share is growing.

Gascón, Los Angeles Police Department Chief Michel Moore and San Gabriel Police Chief Gene Harris, who is also the president of the Los Angeles County Police Chiefs’ Association, joined forces to urge the credit card giants to stop online ghost gun transactions.

In a letter to American Express CEO Stephen Squeri, the law enforcement leaders applauded the decision to not allow its company to be used as payment for a website that facilitated sex-trafficking, the first company to do so.

They appealed to the CEO to do the same to stop the proliferation of ghost gun purchases. They made the same plea to Mastercard and Visa “to persuade you to make a similar responsible decisions with regards to ‘ghost guns,’” said the letter, citing their decisions to not allow payments to Backpage.com.

CNN has requested comment from Amex, Visa, and Mastercard.

Many police departments have reported recovering high numbers of the guns. The Bureau of Alcohol, Tobacco, Firearms and Explosives currently cannot trace most ghost guns because certain frames and receivers – two key components that make a firearm work – purchased online are not classified as firearms by the bureau.

Ghost guns have become an increasing concern for law enforcement and local officials. Last November, the Los Angeles City Council voted unanimously to pass an ordinance that prohibited the “possession, purchase, sale receipt or transport of ‘ghost guns.’”

Moore praised the decision, saying that ghost guns had been linked to 24 murders, eight attempted murders, 20 robberies and 60 assaults with a deadly weapon by the end of last November.

Earlier last year, the city sued Polymer80, a ghost gun manufacturer based in Nevada. At the time of the lawsuit’s filing, Moore stated that the LAPD had recovered more than 700 ghost guns that used parts from Polymer80.

Several cities have reported sharp increases in the number of ghost guns recovered over time – and while they make up a relatively small percentage of the total number of guns recovered by law enforcement, that share is growing.

According to the LAPD, the department recovered 1,921 ghost guns in 2021, more than double the 813 ghost guns recovered in 2020. A representative from the department told CNN the LAPD did not begin separately tracking the number of ghost guns recovered until 2020, further underscoring how quickly ghost guns have proliferated throughout the city.

Police in San Francisco seized 1,089 guns in 2021, about 20% of which were ghost guns. Just five years prior in 2016, ghost guns made up less than 1% of total gun seizures. The city’s police department also reported that nearly half of the firearms recovered in homicide cases in 2020 were ghost guns.