Question: Who was the gunman in the mass shooting at the Pulse nightclub on June 12, 2016?

Context articles: 2016 Orlando nightclub shooting
Westroads Mall shooting
2017 Istanbul nightclub shooting
Omar Mateen
Hands (2016 song)
Mass shooting
Pulse nightclub
Mercaz HaRav massacre
Come Out with Pride
2012 Aurora shooting

On June 12 , 2016, Omar Mateen, a 29-year-old security guard, killed 49 people and wounded 58 others in a terrorist attack/hate crime inside Pulse, a gay nightclub in Orlando, Florida, United States.
 He was shot and killed by Orlando Police Department (OPD) officers after a three-hour standoff.
 Pulse was hosting a "Latin Night" and thus most of the victims were Latinos.
 It was both the deadliest mass shooting by a single shooter and the deadliest incident of violence against LGBT people in United States history.
 It was also the deadliest terrorist attack in the United States since the September 11 attacks in 2001.
The Westroads Mall shooting was a mass shooting that occurred on December 5, 2007, at a Von Maur department store in Westroads Mall in Omaha, Nebraska.
 The gunman, nineteen-year-old Robert A. Hawkins, killed eight people and wounded four, two of them critically, before committing suicide.
 It was the deadliest mass murder in Nebraska since the rampage of Charles Starkweather in 1958.
A mass shooting occurred at a nightclub in the Beşiktaş district of Istanbul, Turkey, on 1 January 2017.
 The attack occurred at about 01:15 FET (UTC+3) at the Reina nightclub in Ortaköy, where hundreds of people were celebrating the New Year.
 At least 39 people were killed and at least 70 were injured in the incident.
 The gunman, Abdulkadir Masharipov, was arrested in the city on January 17, 2017, and Islamic State of Iraq and the Levant claimed credit for his actions.
Omar Mir Seddique (November 16, 1986 - June 12, 2016), also known as Omar Mateen, was an American mass murderer who killed 49 people and wounded 58 others in a mass shooting at the Pulse gay nightclub in Orlando, Florida, on June 12, 2016, before he was killed in a shootout with the local police.
 It was both the deadliest shooting by a single shooter and the deadliest act of violence targeting LGBT people in United States history.
"Hands" is a charity single recorded by various artists for the 2016 Orlando nightclub shooting.
 It was written by Justin Tranter, Julia Michaels, and BloodPop, and produced by the latter and Mark Ronson.
 All of the song's proceeds benefit the families affected during the shooting, and will help cover medical costs and counseling efforts.
 The song was conceived by Tranter, the day after the United States' deadliest mass shooting by a single gunman occurred.
A mass shooting is an incident involving multiple victims of firearms-related violence.
 The United States' Congressional Research Service acknowledges that there is not a broadly accepted definition, and defines a "public mass shooting" as one in which four or more people selected indiscriminately, not including the perpetrator, are killed, echoing the FBI definition of the term "mass murder".
 Another unofficial definition of a mass shooting is an event involving the shooting (not necessarily resulting in death) of four or more people with no cooling-off period.
 Related terms include school shooting and massacre.
 The lack of a single definition can lead to alarmism in the news media, with some reports conflating categories of crimes.
Pulse was a gay bar, dance club, and nightclub in Orlando, Florida, founded in 2004 by Barbara Poma and Ron Legler.
 On June 12, 2016, the club gained international attention as it was the scene of the deadliest mass shooting by a single gunman in U.S. history, and the deadliest terrorist attack on U.S. soil since the events of September 11, 2001.
 Forty-nine people were killed and 53 were injured.
 There are plans to convert the premises into a memorial dedicated to the victims.
The Mercaz HaRav massacre, also called the Mercaz HaRav shooting, was a mass shooting attack that occurred on 6 March 2008, in which a lone Palestinian gunman shot multiple students at the Mercaz HaRav yeshiva, a religious school in Jerusalem, Israel, after which the gunman himself was shot dead.
 Eight students and the perpetrator were killed.
 Eleven more were wounded, five of them placed in serious to critical condition.
Come Out with Pride (COWP) is a 501(c)(3) nonprofit organization based in Orlando, Florida that has organized an annual pride event of the same name since 2005.
 The group was cited as an example of the community activities centered at the Pulse nightclub following a terrorist attack on that location.
On July 20, 2012, a mass shooting occurred inside a Century 16 movie theater in Aurora, Colorado, during a midnight screening of the film "The Dark Knight Rises".
 A gunman, dressed in tactical clothing, set off tear gas grenades and shot into the audience with multiple firearms.
 Twelve people were killed and 70 others were injured, the largest number of casualties in a shooting in the United States until the Orlando nightclub shooting four years later.
 This also was the deadliest shooting in Colorado since the Columbine High School massacre in 1999.
 The sole assailant, James Eagan Holmes, was arrested in his car outside the cinema minutes later.
 Prior to the shooting, Holmes rigged his apartment with homemade explosives, which were defused by a bomb squad a day after the shooting.