Wielding an AR-15 semi-automatic rifle and a handgun, the younger Mateen opened fire at the crowded Pulse Orlando club early Sunday in the deadliest shooting rampage in modern US history.
The 25 year-old woman who recorded the video was among the 49 people killed.
On the same day as the Orlando attack, an IN man armed with three assault rifles and chemicals used to make explosives was arrested IN Southern California and told police he was headed to a West Hollywood gay pride parade, authorities said. Some sources allege he was also armed with a suicide vest. According to the picture now emerging of Mateen, he was a walking time bomb.
Before beginning his attack, Mateen had called the 911 emergency service to pledge his allegiance to Islamic State leader Abu Bakar al-Baghdadi.
ISIS claimed responsibility for the attack, with its news organization announcing the massacre was "carried out by an Islamic State fighter".
Harris, who visited Pulse every other week or so, didn't recall encountering Mateen there, but others did.
Qais Munhazim woke up on Sunday to reports that an Afghan-American gunman had rampaged through a gay nightclub in Orlando, Florida, in an attack apparently motivated by support for militant Islamist groups. In 2006, he registered as a member of the Florida Democratic Party.
Juliette Kayyem, the former assistant secretary for Intergovernmental Affairs at the Department of Homeland Security, told CNN's Jake Tapper that "there appears to be no evidence yet of some directed attack".
Mateen's ex-wife, Sitora Yusufiy, said earlier in the week that he was mentally ill, controlling and abusive.
Nonetheless, Turk, a gay Muslim man who also served as editor-in-chief of the University of California, Los Angeles' Journal of Islamic and Near Eastern Law, said there are Muslims, gay and non-gay, who say Islam's resistance to change is what gives it its appeal as a religion.
Analysts says the pledges are troubling, but it's hard to determine what role, if any, ISIS played in inspiring these attacks.
President Obama called the massacre "an attack of terror and an attack of hate". Franklin Graham regularly preaches in the United States and overseas.
"Our hearts, thoughts, and prayers are with the victims and their families". He did so at age 19, to parents he said are "more tolerant than they are accepting".
At a time when the United States needs unity and consideration most, Americans should not reach for hate of their neighbors because of their religion: They, too, are American, and hurting all the same."And we have to decide if that's the kind of country we want to be".
Mateen bought at least two guns legally within the last week or so, according to Trevor Velinor of the Bureau of Alcohol, Tobacco and Firearms.
Both presidential candidates released statements.
Presumptive Republican nominee Donald Trump has repeated his call for a temporary ban on Muslims entering the country. More to the point, how are we to live?
Chris Stephenson, also a photographer and Harris' business partner, said that whether Mateen's rage came from fundamentalist extremism or self-loathing, the shooting is "tragic either way". It also occurred during "Pride Month", on a weekend when a number of USA cities were hosting homosexual-pride festivals.
Van Horn recalled that Mateen "was trying to pick up people, men". The man was carrying three assault rifles and chemicals used to make explosives.
The events of the day took us on a tour of this struggle -- and showed why it must draw us, ineluctably, together. It is yet unclear what motivated the suspect.
As the tragedy was unfolding, Pulse Orlando posted to its Facebook page: "Everyone get out of Pulse and keep running". Many more victims are yet to be identified, as investigators continue the grisly task. In just the past year, the mass shootings that have captivated America's attention killed 66 Americans, "twice as many fatalities as from Muslim-American terrorism in all 11 years since 9/11", notes Kurzman's team.