A New York State trooper had the law on his side when he shot unarmed escapee David Sweat, apparently in the back, as the convicted killer ran toward a forest near the Canadian border.

State and federal law allows the use of deadly force to prevent an escape if the officer believes the suspect poses a significant threat. Law enforcement experts say this shooting was clear-cut.

"There cannot be any cleaner situation than this one," Maria Haberfeld, head of the law and police science department at John Jay College of Criminal Justice, told the Associated Press . "You cannot shoot any fleeing felon, but certainly you can shoot the one who poses a real threat. There was no reason to believe this person who had killed a police officer before was not posing a real threat."