The US Department of Justice said it had made its investigation into police involvement in the death of George Floyd a “top priority” after thousands took to the streets for a second day of protests in Minneapolis, the Guardian reports.

Prosecutors and investigators from the Federal Bureau of Investigation (FBI) have been assigned as part of a “robust” inquiry into whether the police officers involved had violated federal laws, the department said in a statement.

Some demonstrators peacefully gathered for a second night at the site where Floyd, who was black, died on Monday, after a white police officer knelt on his neck for several minutes, despite Floyd’s cries of “I can’t breathe”.

However, conflict between protesters and police outside a nearby police station soon spiraled into chaos across south Minneapolis, with one man shot dead.

Officers fired flash-bang grenades, rubber bullets and teargas into the crowd. Rocks and other projectiles were thrown at officers at times, the Star Tribune reported.

The governor, Tim Walz, ordered the national guard to the site at around midnight, and St Paul police and the state patrol were also present. Police said they were investigating the shooting death as a homicide and had a suspect in custody.

Jacob Frey, the mayor, told a Star Tribune reporter at around midnight: “Please, please, Minneapolis. We cannot let tragedy beget more tragedy. The activity around Lake and Hiawatha is now unsafe. Please, help us keep the peace.”

Minneapolis’ police chief, Medaria Arradondo, offered an apology on Thursday, talking of a “deficit of hope” in parts of the city after Floyd’s violent death.

“I’m absolutely sorry for the pain, the devastation, trauma,” Arradondo said at a briefing.

But he warned: “I cannot allow criminal acts to threaten the safety and also the trauma that already exist.”

He added: “Crowds got large and became more mobile.” Arradondo said the majority of demonstrators were gathering peacefully but there was a “core group” that was looting and engaging in other criminal acts. 

“The people that were involved in the criminal conduct” were not known to local community leaders, he said. 

The White House press secretary, Kayleigh McEnany, said in a Thursday afternoon briefing that Donald Trump had watched the video of Floyd’s killing and that the president thought it was “egregious, appalling, tragic” and that “he wants justice to be served”.

A Minneapolis police spokesman, John Elder, said at a press conference earlier on Thursday that the shooting death was believed to have occurred after a pawnshop owner had accused the victim of looting his business.

Philonese Floyd, the brother of George Floyd, also appealed for peace on Thursday, adding through sobs: “Everybody has a lot of pain right now, that’s why this is happening. I’m tired of seeing black people dying.”

There was looting on Wednesday evening, though most protests were focused on decrying the death of Floyd, 46. Some large fires were alight on the streets that continued to burn on Thursday morning.

Crowds looted a nearby Target, which at one point was reported to be on fire. Looting spread into dozens of area businesses. 

As they did on Tuesday night, police fired rubber bullets and teargas to try to disperse crowds. The response has been criticized as heavy handed, and city council members pleaded for the police presence to be decreased to try and de-escalate the scenes.

Council member Jeremiah Ellison tweeted: “If the strategy was to keep residents safe, it failed. Prevent property damage, it failed. Why are our officers firing at people from rooftops? Why is MPD [Minneapolis police department] not acting like they work for Minneapolis?”

Protesters began gathering in the early afternoon near Minneapolis’s 3rd police precinct station, in the southern part of the city.

Video of the Floyd incident showed members of the public begging the officer to stop.

The officers had arrested Floyd outside a grocery store after reported use of a counterfeit bill. Four police officers involved have been fired, but Frey and others have called for the officers to face charges for their actions.

About 50 demonstrators gathered outside the home of the Hennepin county attorney, Mike Freeman, on Wednesday night, calling for him to charge the officers with murder, the Star Tribune reported.

About 100 people also gathered at what is believed to be the home of Derek Chauvin, a 19-year veteran of the force who has been identified as the officer who pinned Floyd to the ground and kneeled on his neck.

Earlier on Wednesday, civil rights lawyer Ben Crump, who is representing Floyd’s family, issued a statement urging protesters to stay peaceful and socially distant.

“We cannot sink to the level of our oppressors, and we must not endanger others during this pandemic,” it read.

The civil rights activist Al Sharpton has announced he and Eric Garner’s mother, Gwen Carr, will co-lead a prayer vigil with local religious leaders on Thursday afternoon.

Eric Garner’s death in 2014 at the hands of a New York City police officer was one of several deadly encounters between black people and the police that catalyzed the national Black Lives Matter movement. Garner also gasped “I can’t breathe” as the officer held him in a chokehold shortly before he died.