美国各地为奥兰多枪击案死难者烛光守夜

Thousands of people 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 people died. 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.

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.

Obama declared it an act of terrorism and said the Federal Bureau of Investigation (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 grim president again addressed a stunned nation from the White House, saying 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."

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 said her ex-husband was bipolor. "He was mentally unstable and mentally ill."