HARTFORD, Conn. — Connecticut on Tuesday sued four gun companies it accused of mailing illegal firearm parts with no serial numbers to an undercover state investigator, the latest legal filing by states and cities seeking to crack down on untraceable ghost guns.

Ghost guns are displayed at the headquarters of the San Francisco Police Department in San Francisco in 2019. Ghost guns are firearms without serial numbers. Haven Daley/Associated Press

Announcing the civil lawsuit, Connecticut Attorney General William Tong displayed AR-15 “80% lower receivers” that the companies sent to the investigator. The components can be used to make automatic and semi-automatic rifles.

“Ghost guns are an untraceable menace that exist for one reason – to evade law enforcement and registration,” Tong said. “They are a threat to public safety, and they are illegal in Connecticut.”

The civil lawsuit accuses the companies of violating state consumer protection laws, which carry fines of up to $5,000 per violation. Tong suggested the companies also violated the state’s 2019 criminal ghost gun ban. But he would not comment on any criminal investigation.

Police across the country are seeing a proliferation of ghost guns.

In Hartford, police seized 57 ghost guns last year, up from 21 in 2021 and seven in 2020, Tong said. Connecticut is one of 11 states that regulate the sales and manufacturing of unmarked gun parts, according to the Giffords Law Center to Prevent Gun Violence.

Connecticut joins other states and cities that have sued gun companies over ghost guns, including San Francisco, New York and New York City.

In state court in Hartford, Tong’s office sued Indie Guns of Orlando, Florida; Steel Fox Firearms of DeLand, Florida; Hell Fire Armory of Wilmington, North Carolina; and AR Industries of Orem, Utah.

Indie Guns owner Lawrence Destefano, whose company also is being sued by New York state, New York City and two other cities, called the lawsuits baseless. He said they are an attack on what he called craft gun ownership and a government attempt to track more gun owners.

“The only reason these lawsuits were filed was to force a settlement that requires the defendant to turn over customer data,” he said in a phone interview. “I will never turn over unfettered, blanket, indiscriminate customer data.”

The other three companies did not return messages seeking comment Tuesday.