Obama Calls Florida Shooting ‘Act of Terror’A stern and somber U.S. President Barack Obama took to the podium in the White House briefing room in Washington Sunday to express his condolences to the families of those killed overnight in an Orlando nightclub shooting. It was the 15th time he has had to address the American people after a mass shooting during his tenure in office. Obama said the massacre "marks the most deadly shooting in American history." He said he was briefed at the White House by FBI Director James Comey and added: “Although it’s still early in the investigation, we know enough to say that this was an act of terror and an act of hate.” The president said "no act of terror or hate can change who we are,” but he added that the mass shooting is a "sobering reminder" that an attack on any American is an attack on all of us. Obama addressed the fact that the brutal shooting and hostage taking attack took place at an iconic gay nightclub. Watch: Obama Delivers Statement on Orlando Shooting No media source currently available “This is an especially heartbreaking day for all our friends - our fellow Americans - who are lesbian, gay, bisexual or transgender. The shooter targeted a nightclub where people came together to be with friends, to dance and to sing, and to live. The place where they were attacked is more than a nightclub - it is a place of solidarity and empowerment where people have come together to raise awareness, to speak their minds, and to advocate for their civil rights.” The gunman responsible for the killing of at least 50 people in an Orlando, Florida nightclub early Sunday has been identified as Omar Saddiqui Mateen, a U.S. citizen of Afghan descent who the FBI said committed a crime that is being investigated as an “act of terrorism.” Earlier, Senator Bill Nelson of Florida said intelligence officials informed him the shooter may have pledged allegiance to Islamic State.  Presidential candidates react The presidential candidates also were quick to react to the shootings. The likely Democratic nominee, Hillary Clinton, tweeted: "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." Late in the day, the Clinton campaign and the White House announced they were postponing a planned joint campaign appearance set for Wednesday in the northern state of Wisconsin. No reschedule date was announced. The presumptive Republican nominee, Donald Trump, directly challenged President Obama, tweeting: “Is President Obama going to finally mention the words 'radical Islamic terrorism?' If he doesn't, he should immediately resign in disgrace!” Some Republican leaders and lawmakers have repeatedly called on President Obama to use the words “Islamic terrorism” when speaking about terrorist acts inspired by radical Islamic leaders. The White House has said the president does not want to tarnish an entire religion practiced by millions of innocent people because of heinous acts conducted by a few. House Foreign Affairs Committee chairman Ed Royce issued a statement, saying "My thoughts and prayers are with the victims and all those who lost a loved one today," adding that "Jihadist-driven terrorism has hit America again. House Minority Leader Nancy Pelosi tweeted about the mass shooting on Sunday. ​Senator John McCain also issued a statement extending his condolences.