NEW YORK (Reuters) - Attorneys at a New York City public defenders' office participated in an online video that advocated the killing of police officers, featuring the lyrics "time to start killing these coppers," a city investigation disclosed on Thursday.

The video "Hands Up," which shows singers pointing guns at someone portraying a police officer, was released shortly before the deadly ambush on Dec. 20 of two New York City officers by a gunman angry over police killings of unarmed black men.

Two attorneys with the Bronx Defenders, a city-funded legal service organization, appear in the anti-police video, some of which was filmed at the Bronx office, according to the New York City Department of Investigation report.

The Bronx Defenders posted a message on its website saying it "abhors the use of violence against the police."

"The Bronx Defenders never approved the music video 'Hands Up,' and never saw it before it went online," it said. "We deeply regret any involvement with this video."

Credits at the end of the video cite the Bronx Defenders as its sponsor.

"Advocating the killing of police officers is unacceptable and offensive," Mark Peters, commissioner of the Department of Investigation, said in a statement.

"These attorneys have abysmally failed to meet their obligations to their clients, to the courts and to the city as a whole," he said.

The not-for-profit Bronx Defenders has city contracts of about $20 million for criminal and family defense services, and represents some 35,000 people a year, Peters said.

The findings will be sent to the Mayor's Office of Criminal Justice and the city's Corporation Counsel for possible further action, Peters said.

Mayor Bill de Blasio in a statement called the findings "deeply disturbing."

"Any endorsement of violence against police officers is completely unacceptable and will absolutely not be tolerated," he said.

A deep rift has formed between the mayor and some police over what they see as his failure to support the department during a wave of anti-police protests. Some say he contributed to an anti-police climate.

The mayor demanded the Bronx Defenders take immediate action in response to the findings and said if it does not, the city will do so.

The report also said the Bronx Defenders' executive director, Robin Steinberg, failed to discipline staff members after learning about the video and made misleading statements to officials about their involvement.