Video of the moments after black teenager Michael Brown was fatally shot by a white police officer in Ferguson, Missouri, indicates that a witness on the scene said the unarmed 18-year-old's hands were raised when he was killed.

The cell phone footage, released by CNN, of two construction workers at the scene early last month appears to support accounts by other witnesses that Brown was retreating or surrendering when he was shot by Ferguson police officer Darren Wilson, legal experts said on Friday.

The video shows one of the men raising his hands immediately after the fatal shooting and shouting, "He had his fuckin' hands up."

The man told CNN he heard a gunshot, then another about 30 seconds later. "The cop didn't say get on the ground. He just kept shooting," said the unnamed contractor. He added that he saw Brown's fatal head wound and repeated that the teenager's "hands were up".

The men told CNN they were 50ft (15m) away when Wilson opened fire. The second man said he saw Brown running away from a police car. The teenager "put his hands up", and "the officer was chasing him". Wilson then fired a shot at Brown while his back was turned.

Brown's shooting spurred several days of sometimes violent protests in Ferguson, a predominantly black St Louis suburb, and a nationwide debate about the relationship between young black men and law enforcement.

Police have said Brown assaulted Wilson during a struggle. A local grand jury is considering evidence gathered by St Louis county police for potential criminal charges against Wilson. The Department of Justice (DoJ) is also conducting a civil rights inquiry into Brown's death.

The workers in the video footage are not from Ferguson and were employed by a business from Jefferson county, south of St Louis.

Benjamin Crump, the lawyer for Brown's family, said both workers came forward and told the family their account of the shooting. He described the video as "of paramount significance".

Benjamin Crump, the lawyer for the Brown family, described the new video as 'of paramount significance'. Photograph: Jeff Roberson/AP

"Not because they were not residents of Ferguson, and not because the construction workers were caucasian, but because it is a contemporaneous recording of their immediate actions of what they had just witnessed," Crump said. "It's the best evidence you can have other than a video of the actual shooting itself."

Ed Magee, spokesman for St Louis county prosecutor Bob McCulloch, said the workers were among witnesses interviewed by authorities and were "part of the investigation".

The video would likely be admissible evidence before the grand jury, along with the workers' testimony, said Peter Joy, a professor at Washington University's school of law in St Louis.

"The thing that strikes me is we actually have a film of what's going on and while it's hard to hear the construction workers say what they're saying, you have one construction worker putting his hands up in the air, which appears to be him demonstrating what he's seeing," Joy said.

Lori Lightfoot, an attorney who previously worked as chief administrator for the Chicago police division that oversaw shootings by officers, said the video could be significant but many questions remained: what was the vantage point of the workers? How far away were they from the shooting? "Given the stakes, it's essential that all of these issues be tied down," she said.

The Obama administration has launched a federal investigation into whether police systematically violated the civil rights of people in Ferguson. The DoJ said it would work with the neighbouring St Louis county force, which led the militarised response to protests that followed Brown's killing, on reforming its handling of demonstrations and other practices.

Figures published last year by Missouri's attorney general showed seven black drivers were stopped by police in the city for every white driver, and that 12 times as many searches were carried out on black drivers as white drivers.

It was reported last month that about 13% of Ferguson's officers have faced investigations into alleged use of excessive force – compared to a national average estimated at about 0.5%.