The presumptive Democratic presidential nominee Hillary Clinton on Sunday said she was "devastated" by America's worst mass shooting incident at a gay nightclub in Orlando while her Republican rival Donald Trump said it was an act of "possible terrorism."
Hillary Clinton, the former US secretary of state, offered a tweet signed "-H" to indicate it was from her personally.
"Woke up to hear the devastating news from FL (Florida). 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, Trump said: "Really bad shooting in Orlando. Police investigating possible terrorism. Many people dead and wounded."
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, CNN noted.
Meanwhile, Vermont Senator 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 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," Sanders, who is yet to quit the race for the Democratic Party's presidential nomination race against Hillary, said.
At least 50 people died and another 53 were injured early on Sunday when a heavily-armed gunman opened fire and seized hostages at a gay nightclub in Orlando, Florida, police said in the worst mass shooting in US history.