The parents of an off-duty Chicago police officer who was shot to death three years ago shortly after returning from Iraq announced a federal lawsuit Wednesday against a Mississippi gun shop, saying it was the origin of the weapon a gang member used in the killing.

Holding photographs of their son and struggling with their emotions, Carolyn and Thomas Wortham III recounted at a news conference the slaying of their only son, Thomas Wortham IV. They also said the shooting could have been prevented had the gun shop not ignored clear signs that the man who bought the gun was part of a gun trafficking ring.

“I have to live with what has happened to my son for the rest of my life and I have to get up every day thinking about that [but] I don’t want any other mother or any other family to go through this,” Carolyn Wortham said.

Thomas Wortham IV, 30, had just completed his second tour of duty in Iraq when he was killed May 19, 2010, in front of his parents’ South Side home as he tried to stop four men from stealing his motorcycle. The story gripped Chicago for many weeks.

The lawsuit was filed by the Brady Center to Prevent Gun Violence on behalf of the Worthams in federal court in Mississippi. It alleges that Ed’s Pawn Shop & Salvage Yard “knew or should have known at the time of the sale that the sale of the handgun to Michael Elliott, a straw purchaser, was illegal, as Elliott was conspiring with a gun trafficker.”

The owner of the gun shop, Bruce Edward Archer, did not return calls from The Associated Press.

Elliott, from Mississippi, pleaded guilty to his role as a straw purchaser—a person who buys guns to provide them to convicted felons and others who are prohibited by law from possessing them—and was sentenced to six months in prison.

The lawsuit says that even though Elliott could legally purchase weapons, the fact that he was a first-time buyer who paid $1,500 in cash for three weapons should have raised suspicions.

“A reasonable and law-abiding gun seller would have had doubts about the legality of this sale,” it reads.

The legal action is the latest chapter in a story of a trafficking ring that bought guns in Mississippi and brought them to Chicago to sell to gang members. Aside from Elliott’s conviction, Quawi Gates, who authorities said recruited Mississippi college students to buy guns on his behalf, also pleaded guilty and is serving a 10-year federal prison sentence.

“He was everything we could have hoped for in a son,” Thomas Wortham III said of his son. The father is a retired Chicago police sergeant who shot and killed one of the four men during the attack.

The lawsuit also reinforces Chicago’s leading role in the national gun control debate. After a spike in the number of homicides last year and the January shooting death of 15-year-old honor student Hadiya Pendleton that attracted the attention of the White House, national and local officials have increasingly discussed the thousands of illegal guns that were flooding into Chicago and the hands of violent gang members.

Despite some of the strictest gun control laws in the United States, Chicago Police Superintendent Garry McCarthy and others pointed to statistics that show the city leads the nation in seized illegal weapons—7,400 last year, 2,150 so far this year. McCarthy says those weapons are coming from the suburbs and other states, including Mississippi.

U.S. Sen. Dick Durbin is also trying to crack down on straw purchasers with a bill that would impose prison sentences as long as 25 years. He has pointed to data that shows about 10 percent of guns used in crime in Chicago came from Mississippi.

McCarthy said lawsuits like the one filed Wednesday are “absolutely necessary,” especially after the U.S. Senate’s rejection of expanded background checks.

“I really like this idea,” he said of the lawsuit.