A previously undisclosed video of an unarmed black man who was fatally shot by police has sparked protests after raising further questions about his death.

Michael Brown, 18, was killed in Ferguson, Missouri, in 2014 after he was suspected of taking part in a robbery in which he walked out of a local convenience store with cigarillos.

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His death prompted national protests and raised questions about police brutality towards minorities, sparking an FBI investigation into the actions of police officer Darren Wilson.

During the investigation, local police released security-camera footage of Brown visiting the Ferguson Market and Liquor store and pushing a worker, before taking the cigarillos.

However, they chose not to disclose a second video taken earlier in the day, which shows Brown in a more amicable state, handing employees a small bag, which they pass around and sniff.

Brown was then handed two boxes of cigarillos, which he started to take out of the store, but decided to turn back and hand it back to the employees.

Jason Pollock, a filmmaker who unearthed the video for his new documentary "Stranger Fruit", said Brown exchanged marijuana for cigarillos and returned for them later, undermining the police's version of events.

"He left his items at the store and he went back the next day to pick them up," Pollock said in the documentary. "Mike did not rob the store."

Robert McCulloch, the St Louis County prosecuting attorney, rejected Pollock's account as "pathetic."

"There was no transaction. There was certainly an attempt to barter for these goods, but the store employees had no involvement at all in that," McCulloch told a news conference on Monday (13 March), reported Reuters.

McCulloch said he would release further footage, which he said showed that staff returned the cigarillos, undermining the filmmaker's claim.

Roughly 100 people, who believed the new video was evidence that Brown did not stage a robbery, gathered to protest at the store.

According to St Louis Post-Dispatch, during the protest, one man shot several rounds of ammunition into the sky, while another unsuccessfully tried to set a police car on fire. One woman also punched a police officer in the face, breaking his nose.