Republican U.S. presidential candidate Donald Trump gestures during a campaign rally in Tampa, Florida, June 11, 2016. Scott Audette/Reuters

In the wake of the deadliest shooting in U.S. history, Republican presidential candidate Donald Trump wasted little time suggesting the attack was tied to Islamist militants while his Democratic rival Hillary Clinton initially did not make that connection.

As President Barack Obama Sunday addressed the nation about the incident that killed 50 people and wounded 53 at a gay nightclub in Orlando, Trump assailed him for his longstanding refusal to refer to Islamic State and other extremist groups as in part representing the religion of Islam.

“Is President Obama going to finally mention the words radical Islamic terrorism?” Trump tweeted as the president was speaking. “If he doesn’t he should immediately resign in disgrace.”

Trump in a later tweet claimed the gunman shouted “Allah hu Akbar!” during the attack. U.S. officials as yet have not provided any information that such an incident occurred.

A gunman armed with an assault rifle attacked the Pulse nightclub in Orlando, Florida, in the worst mass shooting in U.S. history.

In his remarks at the White House, Obama said the investigation into the shootings was ongoing and declined to speculate on the motives of the shooter.

“We’ve reached no definitive judgment on the precise motivations of the killer,” Obama said. “The FBI is appropriately investigating this as an act of terrorism.

Similarly, Clinton, in a statement, called the attack an “act of terror” but did not speculate as to the ideology of the gunman, identified by authorities as Florida resident Omar Mateen.

“Law enforcement and intelligence agencies are hard at work, and we will learn more in the hours and days ahead,” said Clinton, the likely Democratic presidential nominee.

Trump earlier had boasted on Twitter the shootings had proved he was “right on radical Islamic terrorism.”

During the campaign for the Nov. 8 presidential election, Trump has proposed a temporary ban on Muslims entering the United States as a means to combat the threat of militant attacks.

“Appreciate the congrats for being right on radical Islamic terrorism, I don’t want congrats, I want toughness & vigilance,” Trump tweeted. “We must be smart.”

A U.S. Rep. Adam Schiff, D-Calif., and a top member of a congressional intelligence committee, said in a statement that according to local law enforcement, the shooter had declared his allegiance to Islamic State. Schiff said that “indicates an ISIS-inspired act of terrorism.”