The protests, vandalism and looting sparked by the recent police shooting of an unarmed black teenager in Missouri underline the tense relations between law enforcement and the black community in the U.S.

Although it has been more than 20 years since a jury acquitted four police officers of using excessive force against Rodney King, prompting riots in Los Angeles, many blacks still perceive they are unfairly targeted by law enforcement.

In the case of Missouri teen Michael Brown, witnesses say he was fatally shot by a white police officer Saturday while walking on a street near his suburban St. Louis home.The shooting has prompted violent protests in Brown's community.

Last month, more accusations of racism followed the death of an unarmed New York black man, who died when a police officer applied a chokehold on him. The man, Eric Garner, had been allegedly selling loose cigarettes.

'Rioting became almost inevitable'

“It’s just about impossible to say whether [relations] have gotten better or worse. Not a month goes by where we don’t see another incident of something happening," said Jon Hurwitz, political science professor at the University of Pittsburgh and author of Justice in America: The Separate Realities of Blacks and Whites

"It tells you something when frustrations have built up to a point where rioting became almost inevitable after this particular shooting in an area where it traditionally has not happened before."

Marchers confront police on the third day of protests against Sunday's police shooting of Michael Brown in Ferguson, Mo. (St. Louis Post-Dispatch/Robert Cohen/Associated Press)

Police forces have tried various strategies to improve relations over the years, including community-oriented policing, attempts to get more police on the streets walking beats and the hiring of more black officers. But it's difficult to say how effective these programs have been.

"I can't say they don’t work. I mean who's to say that things wouldn't have gotten even worse without the intervention of these kinds of programs," Hurwitz said.

But simply hiring minority police officers is not a surefire way of improving the situation, he said.

"I think it’s a positive development, and I think it’s a badly needed development, especially where you have a majority of white police forces in majority African-American communities. I think that’s all for the better. But at the same time, you cant expect that to be a panacea."

The problem, says Hurwitz, is that stereotypes against blacks that associate them with violence continue to persist. Hurwitz said these stereotypes are ingrained across the political spectrum, and that many blacks in the U.S.are just as susceptible​ to stereotypes of blacks as whites.

Generally, whites also perceive their experiences with police officers different than blacks, Hurwitz said, with most whites reporting they have been treated fairly and politely during their encounters.

But blacks talk about being treated rudely and disrespectfully, Hurwitz said, even if their encounters with police were for similar reasons as whites.

Treating suspects based on demeanour

"The report of how those encounters happen, whether it's respectful or disrespectful, whether it's fair or unfair, whether it's polite or impolite, those sorts of things differ dramatically across race," he said.

But Hurwitz said it's a complicated issue, adding that police officers are trained to treat suspects based on their demeanour.

"If the suspect is polite and co-operative, then the police are much less likely to arrest or incarcerate the person. On the other hand, if the suspects are seen as being belligerent or offensive or somehow unco-operative then the police officers are much more likely to take the person in."

It's possible that blacks are less respectful to police, Hurwitz said, but it's also true they have more reasons of being suspicious based on how they have been treated in the past.

Rod Brunson, associate professor at Rutgers University's School of Criminal Justice, said young black men throughout the U.S. have countless stories about their experiences, or their friends and family experiences, with issues of profiling that help shape their views about how they will be treated when encountering police.

He said the research is mixed on whether hiring more minority police officers improves community relations.

But one particular success story has been with the Boston police force, which has partnered with black clergy to forge better relationships with blacks.

The program highlights how success "hinges largely on a police department’s ability to forge and maintain mutually beneficial relationships with organizations that can effectively broker trust between neighbourhood residents and police," a report into the program said.

On Tuesday, nearly a week after Brown's death, a police officer responding to reports of armed men at a protest shot and critically wounded a gun-wielding man near where Brown was killed. The incident follows two nights of violent protests, looting and arrests in the largely black St. Louis suburb. (Mario Anzuoni/Reuters)

Brunson said people have to be willing to try unconventional approaches. Police, for example, should partner with people who have had contact with the criminal justice system, as many have a legitimate voice in the community.

But Brunson said that many police are trying to do their job as best as they can to the best of their abilities and are put in situations where they are in high-crime communities.

"And they are are being asked to do something which may involve them to be engaged in more aggressive policing. So it kind of sets the stage for these tensions to erupt. And both parties share the blame."