Wisconsin officials open investigation into police killing of Corey Ruiz

Officials in the US state of Wisconsin have opened a homicide investigation into the fatal shooting of a man by a police officer.

The police chief in Madison, Wisconsin, told reporters on Thursday that he would cooperate with state investigators. "We will not hide from any aspect of this investigation," John Patterson said.

Police have said that Corey Ruiz, 38, was shot on Wednesday after he pulled out a knife during a struggle with four officers. Bystander footage of the incident has circulated on social media and has sparked protests.

Madison Mayor Satya Rhodes-Conway said the shooting will be "rigorously investigated."

"It does not matter who you are or what your past is; you should not lose your life as a result of an encounter with law enforcement," she said.

According to public records reviewed by US media, Ruiz had a history of misdemeanour and felony convictions including robbery by force, strangulation and drug possession.

Police Chief Patterson said the state Justice Department's Criminal Investigation Division would investigate the shooting, since the incident involved its officers. They will also release the name of the officer who fired their weapon.

Civil rights lawyer Ben Crump, who is representing the family, said at a news conference on Friday that Ruiz's death was "unnecessary, unjustified and unconstitutional", adding "thank God there was video".

"He may have had challenges with mental health - that was diagnosed. He may have had substance abuse issues. He may not have been perfect. But he was her son," Crump said as he stood next to Ruiz's weeping mother, Elsa Ruiz.

Elsa said her son loved his family, adding: "I never thought I would have to bury my baby before they buried me."

His brother, Adam Ruiz, said his death was "sickening" and "inhumane".

"My family needs justice for what they did to him," he said.

None of the officers involved in the incident were wearing body cameras at the time.

The incident came after police were called about an individual trying to enter parked vehicles in the neighbourhood, he said.

Ruiz is said to have fled the scene on a bike before officers tracked him down.

Ruiz fell or was pulled from the bike by four officers, and is accused of pulling out a knife and injuring one officer in the melee.

An officer then tried to control Ruiz with a taser, which was deployed unsuccessfully, Patterson said.

The injured police officer fired three shots at Ruiz, who later died in hospital.

All the officers involved have been placed on administrative leave, Patterson said.

Patterson said that bystander footage from the scene shows "one perspective" of what happened, and added that "not all perspectives are online right now".

Patterson was interrupted by a group of protesters during Thursday's news conference, who questioned the investigation and called for more transparency.

Eleven years ago, a white police officer fatally shot Tony Robinson, a biracial man, on the same street where Ruiz was killed. No charges were filed in that case.