The governor of Louisiana said he has “very serious concerns” about the fatal shooting of a black man by police in Baton Rouge that has set off civil rights protests in the city.

John Bel Edwards promised an independent investigation and said the Justice Department would be involved, indicating the nationwide repercussions of the shooting.

“There should be no doubt in anybody’s mind that this incident is going to be investigated impartially and thoroughly,” he said.

Alton Sterling, who sold CDs outside a convenience store, was shot dead early Tuesday after an altercation with two police officers in the store’s parking lot.

The death was a “horrible tragedy,” Police Chief Carl Dabadie told reporters Wednesday.

“Like you, I am demanding answers,” Dabadie said, adding that there was “a lot we do not understand” while he promised a “thorough, just, transparent, independent investigation” that would be led by the FBI and U.S. Attorney’s office.

According to police, officers were dispatched after receiving a report of an armed man in in front of the store.

A graphic video captured on a mobile phone from a car in the lot and spread online, showed the officers pinning down Sterling and shooting him from close range despite having wrestled him to the ground and appearing to be in control of the situation.

It was not immediately clear whether Sterling had a gun.

In a press conference early Wednesday, Edmond Jordan, a Louisiana state representative and Sterling family attorney, said it was “irrelevant” whether Sterling was in possession of a weapon because he wasn’t wielding one while pinned down.

Quinyetta McMillan, the mother of Sterling’s 15-year-old son, said police acted “unjustly” by killing a man who “simply tried to earn a living and take care of his children”.

Sterling’s son sobbed loudly and broke down before being escorted away from the podium as a visibly distressed McMillan vowed she “will not rest” until justice is served.

A protest was held Tuesday near the site of the shooting and demonstrators filled Sterling's CD table with flowers and sympathy messages.

“We have a wound but we will heal and become whole again,” said Mayor Kip Holden, defending the official institutions in the city including the police department.

The head of the city’s National Association for the Advancement of Colored People or NAACP, a national civil rights organisation, demanded the resignations of the police chief and mayor.

A vigil is planned Wednesday night, as officials are on edge about protests that could erupt into widespread violence, similar to scenes recently played out in the wake of the killings of unarmed black men by police including teenager Michael Brown, 25-year-old Freddie Gray and Eric Garner.

•Text courtesy of Anadolu Agency. Photo shows the victim, Alton Sterling.

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