The city of St. Louis — and the nation — are deeply divided over whether a police officer who killed an unarmed black teenager in Ferguson, Mo., should be charged with a crime.

The law that determines when police can use deadly force generally gives officers considerable leeway in making that split-second decision about whether they need to kill to save themselves or others. Law enforcement experts say the legal standard, established by two Supreme Court rulings from the 1980s, has made it hard for prosecutors to obtain convictions in cases of alleged use of excessive force.

Experts say the high court rulings and Officer Darren Wilson’s account give investigators several reasons to find Wilson justified in shooting to kill 18-year-old Michael Brown.

Wilson said to friends that he shot to defend his life. Deadly force is justified if the officer reasonably believed at that moment that he or others were in imminent danger, and it doesn’t matter whether any danger actually existed.

Witnesses in the Ferguson neighborhood, however, say the officer killed a young man who had turned around and was surrendering, striking him with at least six bullets.

Critics of laws governing the use of force say they give poorly-trained or rogue police officers the benefit of the doubt in cases where they have harmed or killed suspects who actually posed little threat.

“The reality is that the police often use excessive force, including sometimes deadly, and are rarely held criminally or civilly liable — and most police departments have no meaningful internal or external accountability mechanisms,” said Anthony Rothert, legal director of the American Civil Liberties Union of Missouri Foundation.

“As a result, individual officers and departments are unaffected by the use of excessive force, which leave civilians — particularly unarmed men of color — at risk of physical injury, with little the law can do to deter excessive or even egregious force.”

Justice Department statistics show that fewer than 8 percent of complaints of excessive force are upheld against police by their departments. In 2002, the period most recently analyzed, the department noted 26,000 complaints but only 2,000 in which police departments agreed the officer used excessive force.

Police officers are also automatically authorized, based on training in most states, including Missouri, to use deadly force if a suspect tries to grab the officer’s gun. Wilson has told investigators that Brown tried to get his gun during a scuffle in which Brown was reaching into Wilson’s patrol car.

Experts say investigators will look at the “totality of the circumstance” that Wilson was facing. David Klinger, a former police officer and professor of criminology at the University of Missouri-St. Louis, wrote a book on police shootings after he shot a suspect in the line of duty. He said that in Michael Brown’s case, Wilson would likely have taken into account the gun scuffle, any injuries he had sustained, as well as his ability to handle a physical threat from the 18-year-old.

“He’s 295 to 300 pounds. That’s a large man,’’ Klinger said. “Can someone that size overpower you and take your weapon? Those are the sorts of things you have to look at in terms of threats coming from things other than weapons.”

Angry aftermath of the Missouri shooting

The law’s recognition of the police officer’s perspective in shootings is echoed in a favorite saying among police: “Rather be judged by 12 than carried by 6.”

The first of the Supreme Court rulings that still govern law enforcement policies nationwide on the use of deadly force is Tennessee v. Garner. In the 1985 case, the court concluded that police officers could not shoot at a fleeing suspect simply to prevent their escape. They could shoot, however, if they had probable cause to believe the person was a violent felon and posed a significant threat of death or serious harm to the community.

The more overarching decision is the 1989 Graham v. Connor ruling, written by Chief Justice William Rehnquist and at a time when violence against police was rising amid a crack epidemic. In that case, Charlotte diabetic Dethorne Graham had rushed into a convenience store to get orange juice to stop an oncoming insulin attack but left the juice inside and left suddenly because of the long line. He asked a friend who had driven him to the store to instead drive him to another friend’s house for food.

Charlotte city police officer M.S. Connor, suspicious at Graham’s hasty exit, followed him and his friend, stopped them for questioning and didn’t believe Graham’s story about being diabetic. As Connor was checking by radio with the store, Graham got out of his car and passed out briefly. Backup officers arrived, told Graham to shut up and rammed his head into a patrol car while throwing him in the back of it.

Graham sustained minor injuries and argued that the officer’s use of force was excessive. But the Supreme Court found that the officer’s actions were justified because he reasonably believed the force he was using was necessary to prevent or detect a crime in progress.

Edward “Woody” Connette, the lawyer who represented Graham, is still troubled by the broad standard that endorsed police using “outrageous” force with an ill man.

“Somehow there ought to be a higher standard for deadly force,” Connette said. “But I just don’t think it’s possible with the courts we have.”

“The difficult thing is the application of the standard by a jury,” Connette said. “In most parts of the country, unless it’s a minority community or a highly-militarized area, juries tend to identify with police officers. They are our protectors. They are us.”

The county prosecuting attorney who is handling the Michael Brown case, Robert McCulloch, has been widely criticized for his pro-police positions over the years and some of his decisions.

Protests erupted in 2001 when McCulloch concluded two police officers did nothing wrong in another officer-related shooting in the highly segregated north St. Louis County. The drug officers from Dellwood had shot two unarmed black men 21 times behind a Jack in the Box during a botched drug arrest.

The officers initially said the suspects, who had previously been convicted of drug and assault felonies, had tried to escape arrest and then drove toward the officers. But a later federal probe found the men were unarmed and their car had not moved when the police began shooting, killing both suspects.

McCulloch infuriated residents by saying of the suspects, “These guys were bums.” He also drew their ire when he declined to release surveillance tapes of the shooting and other details of his probe.

The National Police Accountability Project has urged police departments to train officers on how to de-escalate a confrontation and consider non-deadly force alternatives whenever possible. The advocacy group also urges that officers get more training in how to assess the actual risks and consider if a person may be acting oddly because they are dazed, in medical distress, mentally-ill or are a non-English speaker.