President Obama on Sunday used the tragic mass shooting in Orlando Sunday to call for gun control measures.

"This massacre is therefore a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, or a house of worship, or a movie theater, or in a nightclub," Obama said during an appearance at the White House. "We have to decide if that's the kind of country we want to be. And to actively do nothing — that's a decision, too."

Obama declared the overnight shooting "an act of terror and an act of hate," but did not characterize the attack as Islamist terrorism.

"We are still learning all the facts," Obama said. "This is an open investigation. We have reached no definitive judgment on the precise motivations of the killer."

Presumptive GOP nominee Donald Trump tweeted that Obama should call the attack "Islamic terrorism" or "resign in disgrace."

The president canceled a planned Wednesday campaign appearance with Hillary Clinton in Green Bay.

Omar Mateen is identified as the shooter. He was reportedly shouting "Allahu Akhbar" during the attack and reportedly had expressed allegiance to the Islamic State. Obama said that law enforcement is working to determine "what if any inspiration or association this killer may have had with terrorist groups."

Obama said that while the killer's precise motivations aren't known, enough is known to characterize the killings as terror.

At least 50 people were killed in what is now the deadliest shooting in U.S. history. The suspect was killed in a gun battle with a SWAT team.