Department of Homeland Security (DHS) Secretary Jeh Johnson said Thursday he won't second-guess the FBI on its past investigations of Omar Mateen, the gunman who shot more than 100 people inside a gay nightclub in Orlando.

“I know how aggressive they are when it comes to their counterterrorism efforts,” Johnson said on MSNBC’s “Morning Joe.”

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“In this case, the shooter was interviewed multiple times, he was investigated, there was a follow-up interview, and the FBI made the judgment based on what they knew at the time that they should dedicate their resources to other investigations.”

Johnson added he admires the FBI’s overall counterterrorism focus.

“I know from working with the FBI daily that their counterterrorism mechanisms are tremendous,” he said. "The FBI does an excellent job of investigating, detecting and preventing terrorist plots in this country. I see that all the time.”

Forty-nine people died and 53 more were injured when Mateen opened fire early Sunday morning at Pulse nightclub.

Reports emerged Monday that the FBI interviewed Mateen three times before he committed the deadliest mass shooting in U.S. history.

Mateen was also purportedly on a federal terrorist watchlist for roughly 10 months in 2013 and 2014 before his ultimate removal in March 2014.