An investigation into residents’ allegations that police officers set off illegal fireworks over a Huntington Beach neighborhood early July 5 has contributed to ongoing tensions between the city’s police chief and the police officers union, according to city officials.

In an Aug. 25 letter, city leaders said police were conducting an “internal affairs investigation of a citizen’s complaint of officer misconduct that occurred outside a union facility.”

The letter did not elaborate about the alleged misconduct but noted that at a July 20 meeting, members of the police union’s leadership “unprofessionally and profanely” demanded that Chief Robert Handy terminate the investigation.

Councilman Erik Peterson confirmed Thursday that the internal investigation centers on complaints that a group of officers set off fireworks in front of their club near Gothard Street and Prodan Drive.

Residents in the adjacent neighborhood, east of Central Park, said they were jolted awake between 2 and 3 a.m. July 5 by large fireworks explosions that sounded like they were being set off above their homes.

Huntington Beach allows residents to discharge legal “safe and sane” fireworks from 10 a.m. to 9 p.m. July 4. It is a misdemeanor under state law to possess, store, use or transport illegal fireworks. Doing so is punishable by a $1,000 fine and jail time.

The Huntington Beach Police Officers’ Assn. criticized Handy in an Aug. 23 news release for being vague about the allegations the department was investigating.

“We take our position of public trust seriously and only ask that we are treated fairly and our constitutional rights and human dignity are protected,” association President Dave Humphreys wrote.

The union added that “there is no discord over being held to account for our actions, including to be disciplined, terminated or prosecuted.”

Handy, who has led the Police Department since 2013, is widely supported by the City Council, despite strained relations with rank-and-file officers over community policing, his efforts to modernize the department with equipment like body cameras and his handling of internal investigations, according to city officials.

Tension between Handy and the Police Officers’ Assn. boiled over last month when union officials announced that officers had taken a 207-11 vote of “no confidence” in the chief.

Humphreys could not be reached for comment Thursday, and Handy did not immediately respond to a phone call seeking comment.

Resident Bill Cuppy said his family and dogs were awakened by the explosions early July 5. The fireworks continued for about 40 minutes, he said.

“I don’t mean little bottle rocket booms, I mean big booms,” he said. “These were Disneyland-level fireworks.”

Huntington Beach police responded to Gothard Street and Prodan Drive at 3:23 a.m. after someone called to report fireworks being set off in the area. The police log states the incident reported was “GOA,” or “gone on arrival.”

About a week later, officers showed up at Cuppy’s door to ask him what had happened that morning.

“They asked questions and I answered them,” Cuppy said.

hannah.fry@latimes.com

Twitter: @HannahFryTCN