The Justice Department's investigation will look into whether the two officers, who have been placed on leave, willfully violated Sterling's civil rights through the use of unreasonable or excessive force.
Federal investigators must meet a high legal burden to bring a civil rights prosecution, establishing that an officer knowingly used unreasonable force under the circumstances and did not simply make a mistake or use poor judgment. Many federal probes conclude without criminal charges.
The shooting has sparked protests in Baton Rouge, and speakers at a Wednesday morning press conference said they want to know why the two officers felt it was necessary to open fire on Sterling.
"Mr. Sterling was not reaching for a weapon. He looks like a man that was actually fighting for his life," said state Rep. Edmond Jordan, an attorney for Sterling's family.
Jordan said police confiscated the video surveillance system from the store, but he said the store owner is confident the video shows the entire confrontation and shooting. Meanwhile, he said the video that someone outside the store made on a cellphone and posted online "certainly speaks for itself."
"It's a horrible thing. It's a horrible thing to happen to him," said Sterling's aunt, Sandra Sterling. "He didn't deserve that."
The mother of Sterling's son, Quinyetta McMillon, trembled as she read a prepared statement at the gathering outside City Hall, where a few dozen protesters and community leaders gathered.
Her son, Cameron, who is 15, broke down in tears and sobbed as he was led away while his mother spoke.
She described Sterling as "a man who simply tried to earn a living to take care of his children."
The video that shows the moments leading to the shooting fueled public anger and helped prompt hundreds to take to the streets to protest Tuesday.
In the video, which appears to be shot from inside a nearby parked car, one of two police officers outside the store can be seen tackling a man in a red shirt and wrestling him to the ground. Then the other officer helps him hold the man down.
At one point someone can be heard saying, "He's got a gun! Gun!" and then one officer on top of the man can be seen pulling his weapon from his holster. After some shouting, what sounds like a gunshot can be heard and the camera pulls away. Then another four shots can be heard. At one point, a person in the vehicle asks, "They shot him?" as a woman can be heard crying.
The Associated Press has not been able to authenticate the video. But the appearance of the store in the video matches the front of the convenience store where the shooting occurred. The man being subdued by police was wearing a red shirt, matching the description given earlier by police.
State Rep. Denise Marcelle of Baton Rouge, who has been at the forefront of the protests, said she has talked to the police chief. She said the chief told her that body cameras fell off both officers during their "tussle" with Sterling. But Marcelle said she saw no evidence of that on the video.
"The footage that we have that's good is from the store. Why haven't they released that video? I don't understand that," Marcelle said.