LARGO — A Pinellas County deputy was arrested Monday for striking a 15-year-old inmate last summer and lying about his motives, marking the fifth dismissal of a county deputy in six months.

Joel De Los Reyes, 51, hit the teenage inmate for "mouthing off," Sheriff Bob Gualtieri told reporters at a news conference Monday. Another deputy, Adrian Nenu, was suspended for not reporting his colleague's use of excessive force.

"I'm sorry it happened," Gualtieri said. "I'm sorry our people did something like this."

De Los Reyes was assigned to supervise a juvenile pod in the south division of the Pinellas County Jail last summer, when internal investigators say he got in a "verbal exchange" with inmate Steven Valentin about whether or not to keep the television on. Valentin was charged as an adult with a federal robbery.

The deputy then removed Valentin from his cell, walked him over to the day room, and pushed him on to a table. He began escorting Valentin toward a door where Nenu was standing; as they reached the door, De Los Reyes struck Valentin with his knee and then his elbow.

"You can see where he just loses it," Gualtieri said as reviewed surveillance video of the Aug. 31 confrontation. "He knees him and then he hits him in the back."

Neither deputy reported the confrontation. When Lieutenant Sara Spiva saw the men walking with Valentin, De Los Reyes told her there had been "horseplay" while Nenu remained silent.

"I think that — I can't say exactly what he was thinking, but he probably felt it was Deputy De Los Reyes' obligation to tell me what had happened. That in no way occurred," Spiva told investigators.

Investigation files say this all happened in a unit for boys who behave well — a "more desirable pod," according to Spiva. The inmates watch television and movies and play video games, and they attend school.

It happened that there was a disciplinary review hearing scheduled for Valentin that night. There, Valentin reported De Los Reyes had hit him. He did not sustain injuries, Gualtieri said.

De Los Reyes initially told investigators that Valentin was noncompliant and that he'd used a "guiding" touch.

"It's a work of fiction and it didn't happen," Gualtieri said. "To think you're going to get away with it is just crazy and stupid."

De Los Reyes later admitted the use of force was not justified, and resigned under investigation. He blamed his own anger management issues and tunnel vision. He was released on bond Monday.

Nenu received a seven-day suspension for not immediately reporting the incident, though Spiva credited him for stopping De Los Reyes from hitting the inmate again.

De Los Reyes had been with the sheriff's office for seven years to Nenu's 25. The sheriff's office is not pursuing child abuse charges because Valentin, who is now 16, was booked as an adult.

Gualtieri emphasized that the jail houses 3,000 inmates, with 750 deputies on hand. "We're a big agency with 3,000 people and things happen," he said.

Still, he said he was disappointed. De Los Reyes is the fifth deputy since June reported to have either have been fired or to have resigned under pressure.

Deputy Elizabeth Kretzer was fired in June for hitting a female inmate in the head after the inmate, angry over a food tray, flipped her off.

In November, Deputy Paul Martin was fired for making inappropriate comments to a 17-year-old girl during a sexual battery investigation. His line of questioning included anal sex, and he asked "are you into girls too," topics irrelevant to the investigation.

Later that month, two deputies resigned under investigation after one of them punched an inmate in the face. Both Mark Capanna and Klaus Reinert failed to report the incident, then denied a physical altercation in fabricated reports. In one variation of the story, Reinert said he had slipped and fallen on a wet floor.

"I hope it stops. I want it to stop," Gualtieri said Monday. "One is too many of these things."

Contact Lisa Gartner at lgartner@tampabay.com. Follow her on Twitter (@lisagartner).