The family of a mentally ill man who died while being restrained, despite pleading with police that he couldn't breathe, is planning to sue over his death.

Footage showing Ben Anthony C de Baca collapse outside a Walmart in New Mexico has sparked fury after officers were seen fist-bumping each other as he lay dead in the street.

The 47-year-old had been detained by officers after he was reported to have been acting erratically.

After allegedly biting a police officer, a spit hood was place over his head.

The bodycam video, taken in September of last year but only recently released, shows him screaming: "Help me please ... you're going to kill me," as he is dragged out of the shopping centre hooded and in handcuffs.

Officers are then seen laughing and joking as he continues to plead with them that he can't breathe. His legs are also shackled and he is seen being pinned face down on the ground with officers' knees on his back.

Minutes later he lay dead.

Two of the arresting cops are then shown in the video fist-bumping each other, in an apparent greeting, as other officers desperately tried to revive C de Baca's limp body.

Medical examiners ruled his death a homicide by "excited delirium (cocaine intoxication) complicated by means of physical restraint".

An investigation by New Mexico's Rio Rancho Police Department cleared officers from the three police agencies involved in his arrest of any wrongdoing.

But C de Baca's family said the death was a senseless act of police brutality and incompetence. Their family attorney, Ahmad Assed, said they are planning a wrongful death lawsuit against the law enforcement agencies involved, the Daily Beast reported.

His wife said C de Baca had a history of mental illness and that he had been "acting very paranoid all week" after doctors changed his medication.

On the day of his death the couple had been waiting in a Bernalillo McDonald's drive-through line when C de Baca began acting irrationally. He rammed into a parked car with his vehicle before fleeing on foot to a nearby Walmart, where he smashed some televisions.

One employee allegedly told police he shouted: "You're all murderers, you killed my kid", prompting workers to call the police.

Officers from the Rio Rancho, Bernalillo and Santa Ana police departments attended the scene and restrained C de Baca, saying: "Stand up or we're going to drag you out, one way or another."

After he allegedly bit an officer on the leg, a cop is heard on camera saying: "This c**t f**k bit the f**k out of me, dude. I had to punch his ass off of me."

Assed said he was alarmed after seeing the footage then show two officers fist-bump each other and talk casually as C de Baca lay dead outside the store.

"That is the most troubling for me," Assed told local news channel KOB 4. "It shows a sign for me that says, 'We are not human. We know someone 10 feet away from us is dead as a result of what we may have done and we have the ability to fist bump one another of top of a dead body.'"

He also questioned the manner in which C de Baca was restrained, including the potentially dangerous way a spit hood, or "spit sock", was used, saying it may have led to him being suffocated.

"I will tell you that spit sock is not to be used for the purpose of trying to stop someone from biting," Assed said. "What we are saying is they misused and misapplied the spit sock on Mr. C de Baca. In doing so, they were very negligent. He is clearly crying out for help. He was clearly indicating to them that he couldn't breathe."

Medical examiners said they could not rule out suffocation as a contributing cause of his death.

If used improperly, a spit sock has the potential to suffocate a suspect – a factor said to be central in the lawsuit being brought by C de Baca's family.