DENVER -- The city of Denver has been notified a lawsuit may be filed over the police shooting last summer that left a mentally disabled teenager dead.

Paul Childs, 15, was shot four times and killed by a Denver police officer who asked him repeatedly to drop his knife.

A letter of intent has been sent by the family of 15-year-old Paul Childs to city officials. That letter's required in Colorado before a lawsuit can be filed.

Officer James Turney shot and killed Childs on July 5. Police were called to the scene on the report that Childs was threatening family members with a knife.

Police said when they arrived, Childs was standing in the doorway of his home, refusing to drop the 13-inch kitchen knife he was holding. The teen's family said he may not have understood police officers' orders and that he was not a threat to any of the officers. The family argues that officers could have easily used a Taser gun instead of lethal force.

The letter, dated Dec. 30, comes from Childs' mother, Helen, and sister, Ashley. It alleges the family suffered "loss of companionship" and intangible emotional injuries by witnessing the shooting -- and said damages may reach $5 million.

The family is represented by the law firm of famed attorney Johnnie Cochran.

City Attorney Cole Finegan said the city has not held discussions with the Childs family about a legal settlement.

The Denver district attorney decided not to charge Turney in the shooting, even though it was the second disabled teen the policeman had shot dead in 18 months. District Attorney Bill Ritter has cleared police in all 70 shootings since he was elected 10 years ago.

Ritter said he had insufficient evidence to prove that Turney's actions amounted to criminal conduct. Ritter said after reviewing videotape and audiotape statements from officers, family members and witnesses at the scene, he concluded that Turney's actions were not unreasonable.

He also said that the officers at the scene were told only that a teen was trying to stab his mother, and not anything of his mentally disabled condition.

Helen and Ashley Childs claim in the letter that Paul lost his life "as a result of the intentional, reckless and/or deliberately indifferent actions" of Denver police officers and emergency-response dispatchers. The family claims that emergency-responders were not properly trained to handle people with mental disabilities.

Their lawsuit can be filed up to two years from the date of the incident.

Last month, Mayor John Hickenlooper unveiled sweeping reforms to improve training, increase the number of tasers available to officers, and review of the use of deadly force in the police department. Hickenlooper also wants the department to train 300 officers in crisis intervention, boosting the number to 500, and appoint a minority recruiter to ensure a more diverse police force.