/ Updated 
By Chris Fuchs

The family of a mentally disabled California man fatally shot by police in December filed a federal lawsuit last week against the City of Long Beach, California, alleging that one of the officers fired his gun eight times even though he and his partner were not in danger.

The lawsuit, dated July 28, accuses Officer Vuong Nguyen of using unjustified deadly force on 39-year-old Mharloun Saycon after responding to a 911 call of a man with a knife inside a casino arcade.

Mharloun Saycon, who was killed last December in an incident with the Long Beach Police DepartmentCourtesy of the Saycon Family

The suit, which names Police Chief Robert G. Luna and 20 defendants whose names were unknown, also accuses the Long Beach Police Department of employing practices of “excessive force and confrontation” when dealing with the mentally disabled.

Nguyen and his partner, whose name was not given in the complaint, were told about Saycon’s mental disability, the lawsuit alleges.

“If defendants had followed protocols, policies, procedures, and training that are standard in their field and required by law, they would have taken less than lethal measures to handle the situation, rather than senselessly inflame it as they did,” reads the lawsuit, filed in U.S. District Court Central District of California.

RELATED: Family of Man Fatally Shot by Police Files Claim Against California City

Long Beach Police Department spokeswoman Nancy Pratt told NBC News that Nguyen is a 15-year veteran of the force. It wasn’t immediately clear if Nguyen was still on active duty.

Pratt deferred comment on the lawsuit to the City Attorney’s Office, which handles litigation against the city. Susie Gomez, a receptionist, told NBC News by phone Monday morning that the City Attorney’s Office had not yet been served with the lawsuit and had no comment.

Saycon, who emigrated from the Philippines in 1985, had been in the Looff's Lite-A-Line Casino Game of Skill for around four and a half hours on Dec. 14 when a manager saw him scratching the surface of a game table with a pocket knife, court papers said.

An excerpt of a lawsuit detailing the incident that led to Mharloun Saycon's death.

The manager asked Saycon to put away the knife, which he did, but later Saycon walked over to a patron he knew, began chatting, and showed off the knife to him, the lawsuit said. Saycon, who was diagnosed with paranoid schizophrenia in his early 20s, did not point the pocket knife at the customer, nor did the customer feel threatened, court papers said.

Several other patrons noticed the knife and alerted a manager, one of whom tried escorting Saycon from the arcade, court papers said. But Saycon stopped at the exit, prompting another manager to phone the police, the lawsuit said.

Their guns drawn, Nguyen and his partner entered the arcade at 10:13 p.m. and encountered Saycon, who, according to the family’s suit, was sitting in a chair with a closed pocket knife in his lap as he watched television.

The officers shouted at Saycon to drop the knife, the lawsuit alleges.

An excerpt of a lawsuit by the Saycon family detailing the alleged failings of the Long Beach Police Department.

“When Mharloun did not immediately respond, the officers tased him, and then one of the officers lunged at Mharloun and beat him in the head with a baton,” according to the lawsuit.

“Only seconds later, Nguyen opened fire from nearly fifteen feet away, shooting into the busy casino, and emptying his clip into Mharloun’s chest, arm, and abdomen,” the suit continues.

“If defendants had followed protocols, policies, procedures, and training that are standard in their field and required by law, they would have taken less than lethal measures to handle the situation, rather than senselessly inflame it as they did.”

After yelling for everyone to leave the arcade, Nguyen and his partner put handcuffs on Saycon as he lay on the ground unresponsive, the lawsuit alleges. First responders and officers who arrived minutes after the shooting administered CPR, but those efforts failed to revive Saycon, court papers said.

In a statement released a day after the shooting, the Long Beach Police Department said the responding officers received an update while en route that Saycon was, in fact, waving a knife inside the arcade and that some customers had left because they feared for their safety.

When officers arrived, they came upon Saycon who was sitting and still armed, police said. Saycon, according to the police statement, did not drop the knife when ordered to do so. Officers subsequently used an "electronic control device" and a baton on Saycon, which police said were both ineffective.

Following the shooting, officers immediately began life-saving measures on Saycon, police said.

The statement added that the Los Angeles County District Attorney's Office was conducting an independent investigation of the shooting. There were limited portions of the incident recorded on video, which authorities were reviewing, according to police.

The Saycon family with lawyers Dan Stormer and Joe Sayas at a press conference announcing their claim.Courtesy NBC Southern California

The Saycons’ lawsuit comes four months after the family first filed a $20 million tort claim against the City of Long Beach in connection with Saycon’s death.

The lawsuit accuses the Long Beach Police Department of failing “to appropriately train and guide its police officers on how to appropriately approach, assess, and interact with disabled individuals like Mharloun, to de-escalate situations involving the mentally disabled, and to seek immediate mental health care for them.”

The family claims Saycon’s constitutional rights were violated and seeks compensatory and punitive damages, as well as attorneys’ fees and costs.

Follow NBC Asian America on Facebook, Twitter, Instagram, and Tumblr.