Tyrone Harris appears in a video released by Missouri police while brandishing a gun before he was shot and badly wounded by officers during Sunday’s protests in Ferguson.
Police said the short clip from a security camera showed the teenager grabbing a gun from his waistband.
Tyrone Harris’s father has said his son was unarmed and was running away from police when he was shot on August 9.
Tyrone Harris, 18 was fired on during protests in Ferguson to mark a year since the killing of another black teenager, Michael Brown.
“The video shows Harris grab a handgun out of his waistband once shots are fired during the protest in the West Florissant corridor,” St Louis’s police department said.
Tyrone Harris’s father said, before the video was released, that his son had been unarmed and was “running for his life”.
Prosecutors have filed 10 charges against Tyrone Harris, including assaulting an officer.
A state of emergency was declared on August 10 as protests continued to mark the anniversary of the fatal shooting of unarmed black teenager Michael Brown by white police officer Darren Wilson.
Protesters staged a sit-in outside the main courthouse in St Louis and gathered along a major highway as part of a day of civil disobedience called by activists.
Police in riot gear forced people off the streets on Monday night. They said 23 people were reportedly detained, but “there were no shots fired, no smoke or tear gas used”.
The state of emergency was still in effect on August 11 but no major protests were reported.
Michael Brown, 18, was shot dead in August 2014 by a white police officer who was later cleared of any wrongdoing by a grand jury and the DoJ.
Michael Brown’s death triggered a wave of protests over alleged racism and the use of excessive force by police officers.