WASHINGTON, June 12 (Xinhua) -- Responding to the deadliest single shooting incident in U.S. history, presumptive Republican presidential nominee Donald Trump said Sunday he is right for being tough on radical Islamic terrorism.
"Appreciate the congrats for being right on radical Islamic terrorism," Trump tweeted. "I don't want congrats, I want toughness & vigilance. We must be smart."
In an earlier tweet after the mass shooting at a gay nightclub in Orlando, Florida that left 50 people dead and 53 injured, Trump called it as a "horrific incident" while "Praying for all the victims & their families."
"When will this stop? When will we get tough, smart & vigilant?" the tweet said.
The brash billionaire was apparently referring to his stance that the U.S. should be tougher on fighting radical Islamic terrorism.
Trump had blasted the Obama administration for being too soft on fighting terrorism as demonstrated by its refusal to even use the term "radical Islamic terrorism."
Trump has also previously stirred up controversy by vowing to impose a temporary ban on the entry to the U.S. by all Muslims.
In a campaign speech in Iowa last November, Trump promised that he will go after the terror group Islamic State (IS) and "bomb the s--- out of them."
No group, including the IS, has so far claimed responsibility for the Sunday shooting.
And it remained unknown if Omar Mateen, the gunman in the Orlando shooting, was motivated by the terror group, though U.S. news media reports said he called 911 before the shooting to swear allegiance to IS.
Presumptive Democratic nominee Hillary Clinton tweeted earlier than Trump that she woke up to hear "the devastating news" from Florida.
"As we wait for more information, my thoughts are with those affected by this horrific act," Clinton said in the tweet.
U.S. authorities are working hard to determine whether the shooting attack is an act of terrorism.
Danny Banks, a spokesman for the Federal Bureau of Investigation (FBI), said that the shooting was being investigated as an act of terrorism.
"Do we consider this an act of terrorism? Absolutely, we are investigating this from all parties' perspective as an act of terrorism," said Banks.
Ronald Hopper, an assistant special FBI agent in charge, said the authorities now had "suggestions that the individual may have leanings toward" ideology held by IS.
But he added that "right now we can't say definitively."
Reports said the parents of Mateen, a 29-year-old U.S.-born citizen, were from Afghanistan.
ABC News quoted U.S. law enforcement officials as saying that Mateen was "on the radar" of U.S. officials for some time, but he was not the target of a specific investigation.