Donald TrumpDonald John TrumpRussian sanctions will boomerang States, cities rethink tax incentives after Amazon HQ2 backlash A Presidents Day perspective on the nature of a free press MORE said Sunday the FBI had a "bad day" when agents investigated Omar Mateen before he attacked an Orlando nightclub, killing 49 people and injuring 53 more.

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The FBI said Mateen was interviewed three separate times about possible ties to terrorism, but it was determined he wasn't a threat.

"I think it's very unusual, and I'm a fan of the FBI, but they had a little bit of a bad day," Trump, the presumptive GOP presidential nominee, said in an interview aired on ABC's "This Week."

An official said the FBI first became aware of Mateen in 2013 when he made "inflammatory comments to coworkers alleging possible terrorist ties."

In the course of that investigation, Mateen was interviewed twice, but the FBI was unable to verify the substance of his comments. 
 
The FBI interviewed Mateen, who was killed in a shootout with police at the nightclub, again in 2014. In that investigation, the FBI was looking into possible ties between Mateen and an American suicide bomber. The FBI determined at the time that the contact did not constitute a threat.

Evidence shows he may have been radicalized and inspired by extremist ideology. 

Mateen allegedly praised the Islamic State in Iraq and Syria and other terrorist groups during the killings.

— This report was updated at 10:29 a.m.

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