Thousands of people, gay and straight, held candlelight vigils in several major cities across the U.S. Sunday night for the 50 people shot dead in the worst mass killing in U.S. history.
Twenty-nine-year-old Omar Saddiqui Mateen -- an American born to Afghan parents -- opened fire in the Pulse nightclub in downtown Orlando, Florida early Sunday morning. Fifty-three people were wounded, some of them gravely.
The nightclub catered to a primarily gay clientele.
Vigils for the shooting victims were held in Orlando itself, as well as in Boston, Chicago, New York, San Francisco and Washington, DC.
In Manhattan, lights on the iconic Empire State Building were turned off in sympathy for the victims. Meanwhile, the spire at One World Trade Center -- near the site of the worst terrorist attack on U.S. soil -- was lit in rainbow colors, the symbol of gay pride.
President Barack Obama ordered U.S. flags to be flown at half-staff in memory of the victims of the worst mass shooting in U.S. history.
Obama declared it an act of terrorism and said the FBI is leading the investigation. He said no effort will be spared to find out what inspired the killer or if he had any links to terrorist groups.
The president again said it is easy for someone to get his hands on a weapon to shoot people in schools, churches, movie theaters and nightclubs.
"We have to decide if that's the kind of country we want to be. And to actively do nothing is a decision as well."View full gallery
He said the murders unite Americans in "grief and outrage."
"This is a sobering reminder that attacks on any American – regardless of race, ethnicity, religion, or sexual orientation – is an attack on all of us and on the fundamental values of equality and dignity that define us as a country," a grim Obama said in a White House statement Sunday morning.
Secretary of Defense Ash Carter said, "If investigators conclude this was an act of terror directed or inspired by ISIL (Islamic State), it will only steel our resolve to defeat this depraved enemy, prevent the spread of its hateful ideology, and defend our people."
Mateen's former wife Sitora Yusufiy said her ex-husband was bipolar. "He was mentally unstable and mentally ill," she told reporters in Boulder, Colorado. While the couple was married for two years, Yusufiy said they were only together for four months because he was abusive.
A Florida gay rights group has established a crowd-funding campaign on gofundme.com to raise money for the victims and their families. Equity Florida says it is working with "a team of attorneys and experts" who have deployed money raised for victims of other U.S. massive shootings "to ensure funds are distributed correctly." More than a million dollars had been raised by early Monday.
WATCH: Witness recalls shooting in Orlando club
Attorney General Loretta Lynch and Homeland Security Secretary Jeh Johnson have both canceled trips to Beijing for cybersecurity talks with Chinese officials.
Florida Governor Rick Scott declared a state of emergency in Orange County, in which the city of Orlando is located.
The city is making public the names of the attack victims after their families have been notified. Orlando citizens lined up for blocks Sunday to give blood for the wounded.
The owner of the Pulse nightclub, Barbara Poma, issued a statement saying she is devastated, and calling her club a place of love and acceptance for the gay community.
Witnesses who scurried from the massacre say the shots came even while the music played on and guests kept dancing.
Mateen traded gunfire with a police officer working extra hours at the club, then left the building and returned, holding a number of hostages for about three hours.
A police SWAT team stormed the club and killed Mateen in a shootout.
Some media reports said Mateen made 911 emergency phone calls during the rampage in which he pledged allegiance to the Islamic State group.
Ronald Hopper of the FBI said Mateen had what he calls minimal ties to an American suicide bomber. Agents investigated him three years ago for possible terrorist ties but could not verify the allegations and closed their probe.
Pope Francis has expressed his “deepest feelings of horror and condemnation” over the Orlando massacre.
The founder of Make Space, an Islamic Center near Washington, DC, told the VOA Afghanistan service "shooting innocent humans is a cowardly act that every Muslim and every human being should condemn."
The Orlando Council on American-Islamic Relations (CAIR) said in a statement, "We condemn this monstrous attack and offer our heartfelt condolences to the families and loved ones of all those killed or injured. The Muslim community joins our fellow Americans in repudiating anyone or any group that would claim to justify or excuse such an appalling act of violence."
The City of Orlando is making public the names of the attack victims as soon as their families have been notified.
Orlando citizens lined up for blocks Sunday to give blood.
WATCH: From the scene of the shooting
On the gay club's Facebook page, a post around 2 a.m. gave early indication of the tragedy that was unfolding.
"Everyone get out of pulse," a page administrator wrote, "and keep running."
Why the club was targeted remains unclear. The shooting comes as many cities around the world celebrate June as Lesbian, Gay, Bisexual, and Transsexual (LGBT) Pride Month.
The Pulse nightclub incident follows another shooting in Orlando by a day.
Singer Christina Grimmie, a 22-year-old YouTube star and one-time contestant of the TV talent show "The Voice," was fatally shot by a man outside her concert late Friday.
Mohammad Habibzada and National Security Correspondent Jeff Seldin contributed to this report. Some material for this report came from AP.