A Florida man was horrified when he realized that he recognized the Orlando shooter — from messages he received from the mass murderer on the gay dating app, Grindr.

Cord Cedeno, a 23-year-old Orlando resident, knew well before crazed killer Omar Mateen opened fire in a crowded nightclub that there was something off with him.

Mateen had tried to flirt with him through Grindr, an app that advertises itself on the Apple store as a way to “find guys close to you for chatting and meeting anywhere in the world.”

“I instantly blocked him because he was very creepy in his messages,” Cedeno told NBC.

Cedeno is one of at least three gay men who claim Mateen used gay dating apps to try to connect with them.

Kevin West, who frequented Pulse, the nightclub where the Sunday morning bloodbath unfolded, told the Los Angeles Times that Mateen messaged him on another app, Jack’d, for about a year.

West had never gone out with Mateen, but said he saw him just hours before the shooting started when he dropped a friend off at a downtown Orlando club.

“He walked directly past me. I said ,’Hey,’ and he turned and said, ‘Hey,’ and nodded his head,” West told the California paper Monday.

Mateen’s ex-wife, Sitora Yusufiy, revealed Monday that she noticed “gay tendencies” in her mate during their short three-month marriage.

Also Monday, a man who worked with Mateen at a Jensen Beach, Fla. Circuit City told the Post that Mateen was fired for “threatening to kill someone,’ in 2004.

Co-Worker David Mitchell said Mateen would carry a knife under his store-issued uniform, and would flash it just to show off.

Additional reporting by Shawn Cohen in Jensen Beach, Fla.