The unidentified man chases a retreating Gaines and punches him, leaving Gaines lying seemingly unconscious in the crosswalk.
Within seconds, two other men descend on Gaines and appear to rifle through his pockets while he lies on the street.
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For the next two minutes, more than a dozen people walk past the fallen Gaines. A few stand over him, but no one appears to make an attempt to help him, while several cars drive carefully around the prone victim.
Then a cab turning the corner where Gaines lies runs him over with a lurch, stopping atop him.
Gaines, who was pinned for at least three minutes, was later pronounced dead at Northwestern Memorial Hospital, according to published reports.
Now his family is suing 7-Eleven, saying workers at the store, which is located in a busy neighborhood north of the city's famous "Loop," did nothing to help him.
“I can’t understand how somebody could just leave him,” Gaines’ aunt, Phyllis Nelson, told WGN-TV. “That’s the part that hurt me so.”
Gaines was killed around 4 a.m. on Feb. 7, 2016, after he stopped in the store to buy some potato chips, the video shows.
Before that, Gaines had been next door at the Mother Hubbard's Sports Pub having a drink with friends.
The bearded, hoodie-clad man seen going after Gaines and punching the bartender has not been identified or arrested, according to the Chicago Police Department.
"This is an active investigation that is still being worked," the CPD said in a statement to NBC News. "The cab driver was not issued any citations on scene."
In the lawsuit, Gaines' family alleges that neither the security guard nor a store clerk ever attempted to help Gaines “in any manner whatsoever and, instead, simply stood on the sidewalk as he lay unconscious and unprotected in the street,” the Chicago Sun-Times reported.
Chicago has for years been grappling with an epidemic of shootings on the impoverished South Side of the nation's third largest city. But Gaines was killed on the Near North Side in a trendy area just a few blocks west of ritzy Michigan Avenue and the Marriott hotel where he worked.
The Gaines case echoes that of the tragic death of Kitty Genovese, who was stabbed to death March 13, 1964 outside her apartment in the New York City borough of Queens. It was later revealed that numerous neighbors heard the screams, but only a couple called police.