Trayvon Martin would have turned 21 on Friday. The 17-year-old Floridian was killed in 2012 by George Zimmerman, a neighborhood watchman who was later acquitted. The controversial case polarized the nation and gave rise to the Black Lives Matter movement.

Three weeks after his 17th birthday, on February 26, Martin was shot and killed in a confrontation with Zimmerman in Sanford, Florida. Martin was visiting his father and had bought iced tea and Skittles at a convenience store. On his way back through a residential neighborhood, he was followed by Zimmerman, who was armed.

Zimmerman, a part-time student and neighborhood watch captain, called 911 to report a “suspicious person,” describing Martin as wearing a hooded sweatshirt (“hoodie”). He ignored the dispatcher’s instructions to stay in his van and confronted Martin.

When the police arrived, Martin was dead from a gunshot wound to the chest, and Zimmerman was bleeding from his nose and back of head, saying he acted in self defense.

Don't forget He is your son nephew grandson god son childhood friend & A just a child walking to the store #TrayvonMartin — Hispanola Ron (@ronemile) February 5, 2016

An all-female jury found Zimmerman not guilty of second degree murder on July 13, 2013.

"You can't put the man in jail even though in our hearts we felt he was guilty. But we had to grab our hearts and put it aside and look at the evidence," one juror told ABC News.

Martin’s death caused outrage with accusations of racial profiling and bias heard across the US and abroad.

On Martin’s birthday, people took to social media to remember the teen and his death.

Martin’s mother Sybrina Fulton has set up the Trayvon Martin Foundation to help other families that have lost their children to violence. It gives scholarships and donates college supplies to poor students.

Join us in celebration of life, peace & positive change at Trayvon Martin Remembrance Dinner #Trayvon21#TMFPWRDpic.twitter.com/5S0OWombdg — TTMFoundation (@TTMFoundation) December 8, 2015

The fourth annual peace walk in Martin’s memory will be held on February 6, across 21 cities in the US.

George Zimmerman did not keep a low profile after the trial. In September 2013, his wife made a 911 call claiming he punched her father and pulled a gun on her. In November of that year, Zimmerman was arrested for threatening and pointing a gun at his girlfriend’s head.

George Zimmerman retweets photo of Trayvon Martin’s corpse http://t.co/y9C1y4glkTpic.twitter.com/gGP1NL7x1v — RT (@RT_com) September 29, 2015

In August of 2015, Zimmerman supported a Florida gun shop owner who banned Muslims from his store. He also called US President Barack Obama a racist and an “ignorant baboon.” In September, Zimmerman retweeted a picture of Martin’s body.

Black Lives Matter

The Black Lives Matter movement began after Zimmerman’s acquittal, taking its name from the social media hashtag #blacklivesmatter. It has since become a large, well-organized movement engaging in social media activism, protests and rallies.

Since Martin’s death, there have been a number of high-profile killings of unarmed African-Americans that have drawn the attention of Black Lives Matter, as well as widespread outrage.

Eric Garner of Staten Island, New York, died from a chokehold in July 2014, in what the medical examiner deemed a homicide. Garner was stopped on suspicion of selling loose cigarettes.

No Eric Garner or Tamir Rice: FBI fails to adequately count civilian deaths by police http://t.co/51c1ipZ8kQpic.twitter.com/mipmip3O9S — RT (@RT_com) October 16, 2015

A grand jury decided not to indict officer Daniel Pantaleo. Following a Department of Justice investigation, the City of New York paid the Garner family $5.9 million in an out of court settlement.

In Ferguson, Missouri, police officer Darren Wilson shot unarmed teenager Michael Brown in August 2014. A grand jury decided not to indict him for the controversial incident.

Remember when Michael Brown got shot and they let his body lay uncovered in the street for hours? Not taking into account it was summertime. — IG/SC: KrysBrown89 (@KrysBrown89) December 28, 2015

In October 2014, 17-year-old Laquan McDonald was killed by Chicago police officer Jason Van Dyke, who shot the teenager 16 times. A year later, the officer was charged with first degree murder after dashcam footage showed him opening fire as McDonald ran away.

In November 2014, 12-year-old Tamir Rice was playing with a toy gun when he was shot and killed by two police officers in Cleveland, Ohio. A grand jury declined to bring charges against officers Timothy Loehmann and Frank Garmback.

What is justice?

One man kills a Horse and gets 35 years

Another kills a 12 yo Black Child #TamirRice and gets ZERO pic.twitter.com/RDrridjXVy — Crystal Johnson (@Crystal1Johnson) February 2, 2016

In April 2015, Walter Scott was shot in the back by Officer Michael Slager in North Charleston, South Carolina. The officer was fired and arrested after a video of the incident emerged, while Scott’s family was paid a settlement.