Donald Trump tweeted that he was "right on radical Islamic terrorism" just hours after news broke of a mass shooting at a gay nightclub in Florida.

"Horrific incident in FL," Trump tweeted at 11:45 a.m. "Praying for all the victims & their families. When will this stop? When will we get tough, smart & vigilant?"

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About an hour later, he tweeted again: "Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart!"

A gunman killed 50 people at Pulse nightclub in Orlando early Sunday. It has become the deadliest mass shooting in U.S. history.

Police killed the shooter, who was identified as Omar S. Mateen, a Florida resident. A top congressman said Mateen may have pledged allegiance to the Islamic State militant group.

U.S. officials cautioned, however, that they had no immediate evidence of any direct connection with Islamic State or any other foreign extremist group.