GRAND PRAIRIE, Texas — Officer Michael Gresham has been with the Arlington Police Department for a little over nine years. But he’ll never forget the radio call he made Friday morning.

“I knew I had to let Grand Prairie know, so I jumped in his Tahoe, grabbed their radio,” Gresham said when describing yesterday’s scene. "'You have an officer down.'"

Gresham was driving on President George Bush Turnpike (161) near Dickey Road with his wife and kids Friday morning. They were headed out of town for a short vacation.

But when the nine-year Arlington police veteran saw Grand Prairie police officer A.J. Castaneda’s Tahoe with two doors open, he knew something wasn’t right.

“As we were passing and we didn’t see the officer anywhere around, that was the first time something clicked, like, this is not good. There’s something not right about this,” Gresham said. “My first initial thought was maybe a drunk driver had struck him, because the red sports car was facing southbound on the northbound side of 161. So as I get out and I’m running, I’m expecting to see him maybe bent down behind his car, in front of the red sports car, somewhere on the bridge.”

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Gresham will never forget what he saw next.

“My heart started to drop at that point. Didn’t know where he was. And that’s when I first noticed two civilians, who had stopped also, and they were peering over the ledge of the bridge, and I was like, 'No, no, please no.' And I look over the bridge, and that’s where I saw Officer Castaneda,” Gresham said.

Castaneda was using a handheld radar outside his patrol SUV when he was hit by a 17-year-old driver around 10:40 a.m. Friday. He was thrown from the highway.

“My heart just immediately went to the floor. Called out to him numerous times. He didn’t respond,” Gresham said.

That’s when the Arlington police officer radioed for help from the Grand Prairie police officer’s vehicle.

“My heart just breaks for Officer Castaneda, his family, the Grand Prairie PD family,” Gresham said. “I’m so sorry, guys. I wish I could’ve gotten there just a couple minutes sooner.”

Castaneda was 38 years old.

Initially, Gresham didn’t know what caused the crash. After radioing for help, he stayed with the 17-year-old driver of the red car.

“He was shaken, but he was okay,” said Gresham. “And he did everything we asked him to do. Was very cooperative and respectful.”

The 17-year-old’s name has not been released yet. No charges have been filed.