Omar Saddiqui Mateen was the gunman who opened fire at a gay nightclub in Orlando, Florida killing at least 50 people, two law enforcement officials told CNN.

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ORLANDO, Fla. — In the midst of the chaos early Sunday morning at Pulse nightclub, the team at News 13 in downtown Orlando was covering the story when a producer took an ominous phone call.

“News 13, this is Matt,” is how Matthew Gentili, a producer at News 13, answered the phone.

Gentili knew something wasn’t right.

“I was receiving phone calls from viewers saying, ‘What’s going on? What’s happening? What’s happening in Orlando?” Gentili recalled Tuesday.

Sharing with viewers the horror taking place just a few blocks down the street became Gentili’s first priority — until a phone call made him pause.

“I will never forget the words he said to me,” Gentili said.

About 45 minutes after the shooting began, the phone rang inside the News 13 newsroom.

“It was at 2:45 a.m. when I had just received the phone call of someone claiming to be the Orlando shooter,” he said. “I answered the phone as I always do: ‘News 13, this is Matt.’ And on the other end, I heard, ‘Do you know about the shooting?’ ”

Gentili said he was aware of the shooting. “Yes, I’m getting calls. I am hearing reports of a shooting,” he replied to the caller, who then cut him off.

“I’m the shooter. It’s me. I am the shooter,” the person on the other end said.

Gentili said he didn’t know what to say. The caller then started to say he committed the shooting for the Islamic State (militant group).

“He did it for ISIS, and he started speaking Arabic,” Gentili said of the early morning phone call. “At the time, I didn’t know what he was saying. He was speaking so fast. But it was … he was speaking fluently. Whatever language he was speaking, he knew it. And he was speaking it very quickly. And that is when I said to him, ‘Sir. Please. Speak in English, please.'”

Gentili said the caller stopped and said: “I did it for ISIS. I did it for the Islamic State.”

Gentili said he asked where the caller was, but the caller said it was “none of my (expletive) business.”

Sitting at his desk in the newsroom, Gentili said there was a long pause before he asked one final question.

“It was silent for a while. I asked him: ‘Is there anything else you want to say?’ ” Gentili said. “He said no and hung up the phone.”

Gentili sat and replayed in his head what had just happened. He heard nothing in the background.

After completing a 12-hour shift, Gentili was interviewed at home by the FBI.

Agents won’t confirm whether Gentili spoke to the gunman, but News 13’s managing editor was able to research the phone number from which the call came, and it matched to Omar Mateen.

“I’m definitely changed,” Gentili said. “When you get a phone call like that, I’m never going to be able to answer the phone again without thinking this is the most serious call I’ll ever get in my life.”