The police shooting of Mike Brown, a black, unarmed Ferguson, Mo., man, is giving the US another tragic opportunity to gauge the limits of justified deadly force with a badge, and what role, if any, American police may be playing in escalating mild encounters into deadly ones.

An unidentified police officer is on administrative leave in the hardscrabble St. Louis, Mo., suburb after killing Mr. Brown on Saturday. An account by Brown's friend, Dorian Johnson, alleges that the officer fired on Brown even as Brown held his hands up in surrender. According to the account, the officer had told Brown and his friend to stop walking in the street and get on the sidewalk when he backed his truck up and got into some kind of altercation with Brown.

Police officials have given scant details, but have suggested that Brown reached for the officer’s gun. Officials say at least one round went off inside the police vehicle – potential evidence of a justified shooting.

Mr. Johnson says the officer grabbed Brown and warned him, “I’ll shoot you,” before doing so. After Brown ran away, injured, he “was giving up in the sense of raising his arms and being subdued,” another witness who says he was watching nearby, Philip Walker, told the Associated Press. The officer, Mr. Walker added, "had his gun raised and started shooting the individual in the chest multiple times." The officer then "stood over him and shot him" after the victim fell on the ground in the fetal position. Officials have not identified the race of the officer, but all but three of the 53-member force are white, and eyewitnesses have said the shooter was white.

The validity of the differing accounts, especially in the absence of video, is now the point of two investigations: one by St. Louis County police, the other by the Department of Justice.

The appearance of injustice, however, sparked two days of protests, rallies, riots, and looting, including the burning of a QuikTrip convenience store. Tensions remain high in Ferguson.

The killing and ensuing aftermath has polarized Americans along racial lines, with some defending the police and citing the riots as evidence of pervasive “thug” lawlessness in America’s rougher neighborhoods. Others see it as another case of excessive use of force against people under suspicion just because of their skin color.

“What do you expect when something is steadily occurring and its hurting the community and nobody is speaking out or doing anything about it?” said Johnson, who was walking with Brown when he was killed, in an exclusive interview on MSNBC. “I feel their anger, I feel their disgust.”

In light of such incidents, policing experts say the public, courts, and lawmakers have started to debate limits to immunity and arguing about policies to recalibrate the role of police.

“Attention is shifting … toward questions about excessive use of force,” says George Ciccariello-Maher, a political scientist at Drexel University in Philadelphia. The public, he says, is taking note of “police impunity, which is essentially that police have not been held accountable for the way they act.”

Brown’s death is the latest in a series of widely-publicized use-of-force scenarios – often made public by the ubiquity of camera phones – that suggest police indifference, even callousness and antipathy, to some of the citizens they’re sworn by oath to protect.

Although not a police shooting, the death of Trayvon Martin became known only after local police failed to charge the shooter, George Zimmerman. (Mr. Zimmerman was subsequently acquitted after being indicted by a special prosecutor.)

Last month, in New York City, a NYPD officer, according to the city medical examiner, used a banned chokehold on Eric Garner, a black man, after he objected to the officers hassling him over the sale of untaxed single cigarettes. Mr. Garner died.

And in recent weeks, Americans also saw troubling footage of a California Highway Patrol officer beating Marlene Pinnock, an unarmed black woman.

The factors that have changed how police interact with citizens include political insulation of police by both Democrats and Republicans, the militarized melding of the war on drugs with the war on terror, “thin blue line” solidarity, and a hard-eyed focus on officer safety, law enforcement experts say. They have had counterproductive societal impacts: The “stop snitching” movement, for instance, can be tied to the “large swaths of Americans who are more afraid of police than criminals,” writes Radley Balko in 2013’s “Rise of the Warrior Cop.”

“The ‘us and everybody else’ sentiment is strong today” among police,” says former Maryland cop Neill Franklin in Mr. Balko’s book.

Statistics from Ferguson raise questions about whether such an “us versus them” mentality exists among police in some of Missouri’s tougher neighborhoods. For example, black people in Ferguson are twice as likely to be stopped by police as white people, according to the Missouri attorney general’s office. According to a report, 92 percent of searches and 80 percent of car stops involved blacks. Sixty-seven percent of Ferguson’s population is black while the police force is almost entirely white.

America is hardly a police state, Balko points out. “There are still good cops … [a] lot of them, [but] we have passed laws and policies that have elevated police officers above the people they serve,” he writes.

Symptomatically, the public has historically shown deference toward the inherent danger of police work, even though being a fisherman, lumberjack, airplane pilot, roofer, or farmer are all statistically more deadly jobs. In addition, general societal attitudes toward black people have soured since 2008, the year President Obama was elected, according to a 2012 study, which concluded that 49 percent of Americans had anti-black sentiments in 2008 versus 56 percent in 2012.

“It shouldn’t surprise us that there’s a tendency to justify and legitimize police behavior and overlook this kind of violence against black youth: They’re people we’re all trained to see as mortal threats already, always potentially guilty,” says Professor Ciccariello-Maher of Drexel.

Police officers can and do face consequences for misuse of force. A police officer in Charlotte, N.C., was last year charged with murder for shooting an injured motorist running toward him asking for help. And a grand jury may weigh murder or manslaughter charges in the Eric Garner death, given that it has been ruled a homicide.

More recently, there has been some pushback in the federal courts against the extent of police immunity when the use of force doesn’t seem commensurate with the situation at hand.

In a 2011 case, the Ninth US Circuit Court of Appeals found that it was unreasonable for a police officer to pepper spray, tase, and baton a black man pulled over for a seatbelt violation because the officer claimed he feared that the lunch-eating man, who had sat down on a curb against the officer’s order, would launch a “broccoli-based attack."

“Whatever such force is ultimately labeled, there is no question that its use against an individual is a sufficiently serious intrusion upon liberty that it must be justified by a commensurately serious state interest,” the court wrote. The officer had several other less intrusive and nonviolent options to deal with the lunch-eating seatbelt violator, the court added.

“What tactics are the officers engaging in before they go in and use force on someone?" former Los Angeles Police Department officer Greg Mayer, who has testified in use-of-force cases, said on NPR Monday. "It’s an evolving issue.”