Presumptive GOP presidential nominee Donald Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE on Sunday thanked his supporters for their praise after an attack at an Orlando, Fla., nightclub that left 50 people dead and many more injured.

"Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance," Trump tweeted Sunday.

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"We must be smart!"

Appreciate the congrats for being right on radical Islamic terrorism, I don't want congrats, I want toughness & vigilance. We must be smart! — Donald J. Trump (@realDonaldTrump) June 12, 2016

The tweet sparked a backlash over its content and timing.

Translating Trump: "20 people are dead. 42 people are injured. But of course, 1st, it's all about Me. Me. Me." Ugh. https://t.co/TDNOqtztvY — Ana Navarro (@ananavarro) June 12, 2016

Bad form, bad punctuation, bad timing, bad to the bone.

The Trump campaign summed up in one tweet. https://t.co/CCATWBNTZr — Jon Ralston (@RalstonReports) June 12, 2016

The attack at the Orlando nightclub early Sunday morning is considered the worst mass shooting incident in U.S. history.

It took place at a gay club during Pride Month.

Police said the shooter, who was killed inside the nightclub during a shootout with police, had an assault-style rifle, a handgun and "some type of device."

An agent from the FBI's Orlando field office said there are "suggestions" the suspect may have had "leanings" toward radical Islamic ideology. But it was not immediately clear if the incident was inspired by foreign terrorism.

Rep. Adam Schiff Adam Bennett SchiffTop Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Overnight Defense: House to vote on military justice bill spurred by Vanessa Guillén death | Biden courts veterans after Trump's military controversies MORE (D-Calif.) said he was told the shooting suspect had pledged allegiance to the Islamic State in Iraq and Syria.

Trump tweeted about the attack earlier in the day, calling it a "horrific incident" and calling for the country to get "tough, smart & vigilant."

As of Sunday, Trump had not changed his schedule for the upcoming week. He is scheduled to give a hard-hitting speech Monday on the Clintons.

— This report was updated at 1:30 p.m.