WASHINGTON -- President Barack Obama called the mass shooting at an LGBT Orlando nightclub Sunday an "act of terror" in remarks to the nation from the White House briefing room.
"We know enough to say this was an act of terror and an act of hate," he said. "The FBI is appropriately investigating this as an act of terror. We will go wherever the facts lead us. ... What is clear is he was a person filled with hatred."
At least 50 people were killed, including the gunman, and 53 more wounded in the what is now the deadliest mass shooting in American history.
Obama said while it could have been any one of our communities, "this is an especially heartbreaking day for our friends who are lesbian, gay, bisexual or transgender."
The president was briefed Sunday morning by several officials, including FBI Director James Comey and Lisa Monaco, Obama's homeland security and counterterrorism adviser, according to the White House. He also ordered American flags to be lowered to half staff to honor the victims.
Vice President Joe Biden has also been briefed on the shooting and canceled a planned trip to Miami to attend a fundraiser for Democratic National Committee chairwoman, Rep. Debbie Wasserman-Schultz.
Biden "offered his prayers for all those killed and injured in the shooting and sends his condolences to all the families and loved ones of the victims," according to a statement from his spokesman.
The political world began to react to the shooting at the LBGT nightclub in the early Sunday morning hours. The LGBT congressional caucus issued a statement saying they were "horrified by the tragic shooting."
"Though details are still emerging, an attack during Pride Month against Pulse, an iconic gathering place for LGBT Floridians, has a particularly insidious impact on our entire community. Our thoughts and prayers are with everyone affected by this tragedy," said Roddy Flynn, executive director of the LGBT Equality Caucus.
Vermont Sen. Bernie Sanders called it "horrific" and "unthinkable" and said it underscores the need for gun control measures.
"Oh, it's horrific, it's unthinkable. And just hopes go out to all those who were shot that they can recover," the Democratic presidential contender said Sunday on NBC's "Meet the Press."
"And I've got to tell you, 25 years ago, I believe that in this country, we should not be selling automatic weapons which are designed to kill people," Sanders said. "We have got to do everything that we can on top of that to make sure that guns do not fall into the hands of people who should not have them, criminals, people who are mentally ill. So that struggles continues."
Hillary Clinton, the presumptive Democratic presidential nominee, offered a tweet signed "-H" to indicate it was from her personally.
"Woke up to hear the devastating news from FL. As we wait for more information, my thoughts are with those affected by this horrific act," Clinton wrote.
Donald Trump, the presumptive Republican presidential nominee, referred to the shooting as "possible terrorism."
In a tweet Sunday morning, Trump said: "Really bad shooting in Orlando. Police investigating possible terrorism. Many people dead and wounded."
He later added: "Horrific incident in FL. Praying for all the victims & their families. When will this stop? When will we get tough, smart & vigilant?"
Trump also tweeted: "Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!"
And just before the president spoke, Trump tweeted: "Is President Obama going to finally mention the words radical Islamic terrorism? If he doesn't he should immediately resign in disgrace!"
He was one of the first politicians to react amid reports that federal and state law enforcement officials were investigating the mass shooting as an act of terrorism.
His reference to "possible terrorism" was a much more measured reaction than Trump issued a month ago after the EgyptAir flight crash, which he tweeted "looks like yet another terrorist attack" before authorities had addressed possible causes.
Florida Gov. Rick Scott pledged to devote "every resource available" to helping after the shooting.
"My prayers are with the victims' families & all those affected by the shooting in Orlando. We will devote every resource available to assist," Scott tweeted.
Republican National Committee Chairman Reince Priebus tweeted: "Our thoughts and prayers are with the victims of the tragic Orlando attack and their families."
Florida Sen. Marco Rubio tweeted: "Our prayers are with those injured and killed early this morning in horrifying act of terror in Orlando."
A top Senate Democrat, Chuck Schumer of New York, wrote on Twitter: "Horrified and saddened by the appalling attack at Orlando LGBT nightclub. Praying for the victims and their families."
New York Gov. Andrew Cuomo said: "Saddened to hear of the senseless mass shooting in Orlando. My thoughts are with the victims, the injured and their loved ones," he tweeted.AlertMe