Despite repeated calls for change, police killings of black men and boys, many of whom were unarmed, have continued in the U.S., sparking outrage around the country.

Two more men were killed by officers this week — Alton Sterling in Louisiana and Philando Castile in Minnesota. 

The shootings have inflamed already strained relationships between many communities and their police forces. Protests were held Thursday in New York, Minnesota, Atlanta, Chicago and Philadelphia in reaction to this week's shootings. In Dallas, a protest erupted into chaos after snipers shot at police officers, killing several of them.

Numbers draw a grim picture

The problem of black men being killed by police is so prominent in the U.S. that President Barack Obama called it "an American issue," in a speech Thursday.

"When incidents like this occur, there's a big chunk of our fellow citizenry that feels as if, because of the color of their skin, they are not being treated the same, and that hurts, and that should trouble all of us," he said. 

"This is not just a black issue, it's not just a Hispanic issue. This is an American issue that we all should care about."

He said the shootings are symptoms of a "broader set of racial disparities" in the country's justice system that aren't being fixed quickly enough.

Pro Publica analysis found there were 1,217 deadly police shootings between 2010 and 2012 in the U.S., according to federal data. Their research found that black people were 21 times more likely to be killed by police than white people. 

Another project, called Mapping Police Violence, was compiled by data scientists, teachers and activists, including well-known activist DeRay McKesson. Among their findings are that black people are disproportionately killed by police in the country's largest cities: in 2015, 41 per cent of people killed were black, despite making up only 20 per cent of the population in those cities.

The Guardian's project, The Counted, found there have been 136 killings of black people by law enforcement in the U.S. in 2016, and 306 in 2015. Most of the victims were men. 

Below are some of the cases that led to protests, charges against the police officers involved, and sometimes, hefty settlements for the families of the victims. 

Jamar Clark, 24, Minneapolis, Minn., November 2015

Killings by Police Minneapolis

(Javille Burns/Associated Press)

Police said Clark was a suspect in an assault and was interfering with the assault victim's treatment. Two police officers were trying to restrain him when he allegedly resisted and ended up on the ground with one officer. One of the officers then fatally shot Clark.

The officer who shot him said he believed Clark was trying to grab his gun.

Clark's death sparked days of protests in Minneapolis.

During a federal investigation, there were conflicting reports as to whether Clark was handcuffed or not when he was shot. The investigators determined that Clark was not handcuffed. 

In March, the U.S. Attorney's Office in Minnesota said it would not bring charges against the officers involved, prompting more outrage in the community.

Killings By Police Minneapolis

Demonstrators raise their fists in the air in Minneapolis after hearing the decision that no charges would be filed against two police officers involved in Clark's fatal shooting. (Jim Mone/Associated Press)

Freddie Gray, 25, Baltimore, April 2015

freddie gray

(Family photo/Murphy, Falcon & Murphy) (Murphy, Falcon & Murphy)

Gray was transported by police in a van after allegedly being caught with an illegal switchblade. During his ride in the van to the police station, he suffered injuries to his spinal cord and went into a coma. He died several days later. 

His death set off protests and violence. 

Six officers were charged after Gray's death, with the charges ranging from manslaughter to illegal arrest. Four officers have been acquitted, and the trial for the fifth and most senior-ranked officer began this week. 

Baltimore reached a $6.4-million US settlement with Gray's family. 

USA-POLICE/BALTIMORE

Residents carrying signs stage a 'die-in' rally on a street of west Baltimore on May 1, 2015, following Gray's death. (Adrees Latif/Reuters)

Walter Scott, 50, North Charleston, S.C., April 2015

Police Officer Fatal Shooting

(Scott Family/Associated Press)

Scott was shot eight times from behind as he ran from a police officer after a traffic stop.

The officer, Michael Slager, was fired from the police force and indicted on a state murder charge. He will go to trial for murder in October. 

A bystander caught the shooting on video, which served to discredit Slager's initial reason for why he shot Scott. The officer said Slager had grabbed his Taser and then came towards him, which the video proved to be false.

The City of North Charleston awarded Scott's family $6.5 million in a settlement, which was the largest in the state's history.

Eric Garner, 43, New York, July 2014

Police Chokehold Death

(Family photo via National Action Network/Associated Press) (Family photo via National Action Network/Associated Press)

Garner was stopped on the street by police, who suspected him of selling illegal cigarettes. Garner said he was not selling cigarettes, and that he was tired of being harassed by police. An officer began to arrest Garner, putting him in a chokehold and pulling him to the ground.

The officer took his arm from around Garner's neck, and he and other officers turned Garner onto his stomach while they arrested him. While on his stomach on the ground, Garner repeated 11 times that he could not breathe. He lost consciousness. Officers did not perform CPR on Garner, but instead waited for an ambulance to arrive.

He died an hour later in hospital.

The arrest was captured on video by a bystander. Garner's words, 'I can't breathe,' became a slogan for many protesting the police's actions.

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Logan Browning, with duct tape over her mouth, joins demonstrators protesting against police violence in Hollywood on Dec. 6, 2014. (Patrick T. Fallon/Reuters)

A grand jury decided not to indict the officer who put Garner in a chokehold. 

Several months after Garner's death, the city announced that its police force would undergo retraining.

Garner's family was awarded $5.9 million in a settlement. 

Tamir Rice, 12, Cleveland, Ohio, November 2014

Tamir Rice

(Facebook) (Facebook)

Rice was killed by a police officer responding to a 911 call about what the caller said was a "probably fake" gun being brandished outside a recreation centre.

An officer pulled up in his vehicle to the scene so quickly that the vehicle skidded, and then shot Rice within two seconds of opening the car door.

Rice was not given first aid until an FBI agent, trained as a paramedic, arrived four minutes later. Rice died the next day.

In December, a grand jury declined to bring charges against the officers involved.

Cleveland awarded Rice's family a $6-million settlement.

Michael Brown, 18, Ferguson, Mo., August 2014

Michael Brown

(Facebook)

Brown and a friend were leaving a convenience store when they were confronted by a police officer. Brown and the officer were involved in some kind of scuffle, followed by gunshots. Brown died at the scene.

Brown's death sparked intense public outrage, and large protests rocked the St. Louis suburb of Ferguson for days. The state's governor called a state of emergency.

A grand jury decided not to charge the officer who shot Brown.

The U.S. Justice Department investigated the Ferguson Police Department over how, among other things, officers use force when arresting suspects.

The family has filed a wrongful death suit against the city of Ferguson. It is scheduled to go to court in October.

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In August 2015, Amarion Allen, 11, is seen standing in front of a police line after a day of peaceful events commemorating the one-year-anniversary of the fatal shooting of Michael Brown by a white officer. (Rick Wilking/Reuters)

Trayvon Martin, 17, Sanford, Fla., February 2012

Black Lives Matter Trayvon Martin George Zimmerman

Americans angry at the acquittal of George Zimmerman in the death of black teen Trayvon Martin protest in Los Angeles on July 14, 2013. (Robyn Beck/AFP/Getty)

While not police, a neighbourhood watch volunteer, George Zimmerman, killed Martin, who was unarmed. Martin had been walking through the gated community where he was staying, and in which Zimmerman lived. 

Zimmerman, who said he was defending himself when he killed Martin, was found not guilty by a jury.

The case was highly publicized. Many rallied around it, using the symbol of the hoodie, which Martin was wearing when he was killed, to point out dangerous racial profiling. 

Martin's family settled a wrongful death lawsuit against the homeowners' association in the gated community where Martin was shot. The settlement amount was not made public.