Abel Esquivel was shot to death in August at 26th Street and South Van Ness, just a block from his home.
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"He had just gotten off work. It was 2 in the morning. He did not know his assailants," said the victim's family attorney, Alison Cordova.
She says the shooter and two others had been on a crime spree -- three shootings and five robberies in one night.
"That was enabled solely by a police officer who apparently is a decorated veteran who failed to secure his weapon in his vehicle and leaving it with ammunition," Cordova said.
Cordova represents Esquivel's family. She says the suspected shooter, Erick Pineda, stole the gun from a San Francisco police officer's personal car.
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The suspect was also wearing an ICE "tracking device" strapped to his ankle. "That monitor, in fact, was the evidence they are using to tie him to the three shootings and five robberies that occurred," Cordova said.
Esquivel's family is suing the city, its police department and the owner of the gun, Officer Marvin Cabuntala.
The complaint will be filed Wednesday.
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"They want the message to be loud and clear that law enforcement officers need to start securing their weapons properly," Cordova said.
And that law applies to both private citizens and law enforcement. "You have to either lock the firearm in the trunk or you have to lock it in a container that's out of plain view," Cordova said.
Police tell ABC7 News they are investigating the circumstances surrounding the stolen gun.
Meantime, they say the officer is still on duty pending the outcome of that probe.
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ABC7news has learned that the family of man fatally shot in August in Mission by suspect with gun stolen from SF cop is suing City. pic.twitter.com/ulRilB239a— Vic Lee (@vicleeabc7) February 14, 2018