In New York State in 1968: A police officer may not draw his gun even....
....upon entering the scene of a reported armed robbery, even if the suspect has drawn his gun -- unless -- the suspect points his gun at the officer. Which newly revised deadly force rule quickly got an officer killed in Queens, New York; leading a quick legislative reversal. The equally cruel rule it succeeded had authorized police officers to fire a shot in the air to warn -- any -- fleeing suspect, and then shoot to kill (you will observe this rule re-enacted on some episodes of the late 50s, early 60s series "Naked City").
A similarly deadly to law enforcement rule change seems to have fallen de facto from the sky -- across the entire nation -- in the form of the conviction and draconian sentencing of two U.S. border patrol guards in El Paso federal court for shooting a fleeing drug smuggler whom officers believed -- but admitted were not sure -- had a gun in his hand.
The El Paso, Texas U.S. Attorney's Office's took the combination of federal civil rights law and a U.S. Supreme Court finding that "it is a violation of someone's Fourth Amendment rights to shoot [someone] in the back while fleeing if you don't know who they are and/or if you don't know they have a weapon" as a federal civil rights formula for prosecuting the (honest enough to admit they were not sure about the gun) border patrol officers.
The officers believed they acted in fear for their lives as they were chasing a suspect who had just left one of them floored and bloody in the act of breaking free (not your typical illegal job seeker) and kept looking over his shoulder while running with an object in hand, at one point turning towards them and pointing the "shiny object" they took for a gun...
....according to the convicted officers at least. The prosecution-immunized drug smuggler -- 800 pounds of marijuana were subsequently found in his van -- told a different tale under oath. He escaped at the time, making it impossible to absolutely prove or disprove possession of a gun.
Justice Oliver Wendell Holmes famously declared that "we cannot expect calm deliberation in the face of an upraised knife." To which we may add the modern day knowledge that adrenalin can diminish you judgment every bit as much as alcohol.
Back in the late 70s when I was driving for a car service in the Bronx I had more than one almost accident with police cars -- not chasing a suspect -- but rather whose drivers had ALREADY made an arrest and were so pumped that they blew red lights forgetting lights and sirens.
If the El Paso decision holds up there should theoretically be no defense for police officers who fire when they think a suspect is even reaching for a gun -- if they were not sure. If the El Paso case holds the FBI should theoretically be prepared to investigate every police shooting in every state that fits the newly coined mis-understanding about the need for certainty about the suspect possessing a gun. Back turned doesn't mean a thing: the quick and the accurate are out there.
Last -- and perhaps most importantly: Justice White's dictum -- on which the prosecution theory lies -- did not comprehensively rule on the aspects of "imminent danger" as far as I can see. Justice White defended his opinion at the time that by explaining "It is better for all suspects to escape than for all suspects to be killed." That sounds to me like a rule that finds society's need to apprehend the suspect of less weight than the suspect's Fourth Amendment rights -- not a rule that says finds imminent danger to the officer's life automatically outweighed by the suspects rights.