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," said Maria Haberfeld, head of the law and police science department at John Jay College of Criminal Justice. "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 posting a real threat."

The same legal reasoning applied to the killing of his accomplice, Richard Matt, who was shot three times in the head on Friday. Unlike Sweat, he was found with a weapon — a 20-gauge shotgun.

Sweat eluded capture for two more days, until he ran across Sgt. Jay Cook, a 21-year veteran who was part of the huge manhunt for the two convicted murderers who used power tools to break out of the Clinton Correctional Facility in Dannemora on June 6.

Cook was alone in his car when he spotted someone walking along the side of the road less than 2 miles from the Canadian border. He got out of his car, approached the man and said: "Hey, come over here," New York State Police Superintendent Joseph D'Amico said at a news conference.

Sweat fled, and Cook chased him, firing twice, fearing he would lose the fugitive in the trees, officials said. Photos appeared to show emergency crews tending to Sweat's back as he sat bloodied in a field. He was listed in serious condition Monday.

A 1986 U.S. Supreme Court case known as Tennessee v. Garner laid out how force can be used to capture a fleeing suspect: Deadly force can't be used to prevent escape unless "the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others."

New York state law also allows for deadly force if a dangerous convict is escaping from a detention facility — which is why armed guards may be stationed in towers at prisons.

Sweat's shooting differs from the recent killing of Walter Scott in South Carolina because Scott was stopped for a minor traffic infraction, was unarmed and was not considered a dangerous criminal, experts said. The white officer who shot the black man five times in the back has been charged with murder.

"But these prisoners, they've gone through the justice system," said Bill Johnson, head of the National Association of Police Organizations. Because they were convicted, "they're not presumed to be an innocent citizen walking down the street."

Some people online questioned the decision to fire, but many lauded the trooper. Gov. Andrew Cuomo called Cook a hero and congratulated him on his "great police work." Onlookers erupted in cheers when the ambulance carrying Sweat passed by.

Carl Thomas lives about a half-mile from where Sweat was captured and said troopers definitely made the right decision by killing Matta and shooting Sweat.

"If he would've got in the woods right there, there would be no chance" to catch him, Thomas said of Sweat.

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Associated Press Writer Michael Balsamo in Malone, N.Y., contributed to this report.