Daniel Herbert told CNN that Officer Jason Van Dyke arrived at the scene 18 minutes after a suspect carrying a knife was reported to have threatened businesses and vandalised police cruisers.
Prosecutors said that 30 seconds after he arrived, Van Dyke shot 17-year-old Laquan McDonald 16 times.
“The reason my client, Jason, fired his weapon that evening, back in October, 2014, is that he truly was in fear for his life, as well as the lives of his fellow police officers,” Herbert said.
The Chicago shooting charges follow a year of unrest across the US, over police shootings of black men, sparked by the August, 2014 killing of unarmed Michael Brown, 18, in Ferguson, Missouri.
The video of the McDonald shooting, filmed by a camera mounted on the dashboard of a police car and made public on Tuesday under orders from a judge, prompted mostly peaceful street demonstrations in Chicago.
WATCH LIVE: Laquan McDonald protests continue in Chicago https://t.co/AssSqLN98D— Fox News (@FoxNews) November 25, 2015
Van Dyke, the first Chicago police officer in decades to face a murder charge for an on-duty incident, was charged hours before the video was released.
Authorities said McDonald was carrying a pocket knife and had the hallucinogenic drug, PCP, in his system. Prosecutors said Van Dyke fired the 16 shots six seconds after emerging from his patrol car, emptying his gun and preparing to reload.
Herbert told CNN his client was afraid McDonald was going to attack him with a knife. Herbert said the video was not an indicator of his client’s guilt.
“Video, by its nature, is two-dimensional. It distorts images. So what appears to be clear on a video sometimes is not always that clear,” Herbert said.
The release of the video has been controversial.
Civil rights activist, Al Sharpton, criticised Chicago city officials for blocking the video, which was made public only after an independent journalist filed a Freedom of Information Act lawsuit.
Sharpton, speaking on MSNBC’s Morning Joe programme, called for a special prosecutor to handle the case, “because the politics here need to be investigated”.
The Chicago Police Department had argued that releasing the video would taint multiple investigations.
“Investigations of police shootings and misconduct are highly complex matters that carry with them very unique legal issues, that must be fully examined and taken into consideration,” Cook County prosecutor, Anita Alvarez, said on Tuesday.
Presidential candidate, Jeb Bush, said it was hard to be a police officer, but “when they do what appears to have happened here, they should be charged.”