The second fatal police shooting of a black man in two days has sparked outrage across the United States after the incident was live streamed on Facebook by the victim's girlfriend. 

The killing of Philando Castile, 32, who was shot by a police officer after a traffic stop in Minnesota on Wednesday, prompted Governor Mark Dayton to call on the US Department of Justice to begin an investigation, while saying there was "every indication" that the police conduct was "way in excess".

The killing took place days after the Louisiana police slaying of Alton Sterling, who was shot dead by officers on Tuesday in Baton Rouge while selling mixtapes. 

"Would this have happened if the driver and the passengers were white? I don't think it would have," Governor Dayton said on Thursday, adding that a state investigation was already under way. 

"This kind of racism exists and it's incumbent on all of us to vow and ensure that it doesn't continue to happen."   

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In a Facebook post on Thursday, President Barack Obama said he and his wife Michelle shared the "anger, frustration and grief" many Americans feel.

"All Americans should be deeply troubled by the fatal shootings ... We've seen such tragedies far too many times, and our hearts go out to the families and communities who've suffered such a painful loss."

'This death is not new'

The use of force by police against African Americans in cities from Ferguson, Missouri, to Baltimore and New York has sparked periodic and sometimes violent protests in the past two years, and has spawned a movement called Black Lives Matter.

Anger has intensified in incidents when the officers involved were not acquitted or not charged at all.

"This death is not new. Black death at the hands of the state, at the hands of police, at the hands of racist vigilantes is not new. It is a part of the blood and the structure of this country," Aislinn Pulley, an activist with Black Lives Matter, told Al Jazeera.

"What is new is that we are able to expose it and we are able to expose it in real time, and we have never had the ability to do that."

Reynolds' video depicted a police officer outside the car pointing a gun. She described what was going on, sometimes speaking calmly to the police officer, sometimes with her voice rising as she feared Castile was dying.

Reynolds said Castile, 32, was shot after police pulled their car over, citing a broken tail light. "Nothing within his body language said, 'Kill me, I want to be dead'," she said on Thursday.

St Paul Public Schools said in a statement that Castile had worked for the district since 2002, and that colleagues were mourning a cheerful "team player who maintained great relationships with staff and students".

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Dozens of protesters gathered at the governor's mansion in St Paul, about 10 miles (15 km) southeast of the scene of the incident, where the governor spoke at a news conference with Reynolds and civil rights activists.

Aftermath of US police killing streamed on Facebook

As Reynolds spoke, people shouted "murder" and called for the arrest of the police officer involved.

She said police had not even tried to check if her boyfriend was alive after they shot him, and it had taken at least 15 minutes for paramedics to arrive.

"Not one shot, not two shots, not three shots, but five shots," she said at the news conference. "They did not check for a pulse at the scene of the crime."

Demonstrations over the deaths of Castile, Sterling and other black men killed by police were planned in St Paul, New York, Chicago and several smaller cities on Thursday evening, according to organisers posting on social media.

Other rallies, including one in Atlanta, were planned for Friday. Protests as far away as London were being discussed on Twitter for the weekend.

Source: Al Jazeera and agencies