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A Texas sheriff's deputy working a second job was killed, and two colleagues hurt, in an apparent ambush outside of a Houston nightclub, officials said.

"We believe they were ambushed, shot from behind," said James Jones, executive assistant chief of the Houston Police Department.

The Harris County deputies were working "police-related extra jobs" at the 44 North Bar and Lounge in northern Houston when the fatal incident unfolded around 2:15 a.m. Saturday outside a nightclub on the North Freeway, Jones said.

"This is a terrible time," Jones said.

The trio went outside to check a disturbance and were trying to detain a suspect when they were shot, Jones said.

"Turns out it may have been a robbery that they were intervening in," he noted, adding the suspect is believed to be a young Hispanic male. Jones said it wasn't clear if they were able to return fire.

One deputy was killed at the scene. A second was shot in the back and was in surgery early Saturday. The third was shot in the foot and was awaiting surgery, said Harris County Precinct 4 Constable Mark Herman.

One "person of interest" was detained at the scene, but Jones said it wasn't yet clear if that person was a suspect or a witness.

"There are a lot of unknowns at this point," he said.

The incident is a "tough time for Precinct 4," Herman told reporters Saturday morning. "I've got a dad there, I've got a wife there, and they've got a deceased son and husband."

The prognosis for the deputy shot in the back was unknown, but Herman said should both of the injured survive, "this incident's going to affect them for the rest of their lives."

"We hope to have a suspect in custody soon. I hope for swift and quick justice for that individual because he ambushed my deputies," Herman said.

"It never gets any easier," Jones said, noting that the Houston Police Chief Troy Finner was not on the scene because he is in Washington for National Police Weekend, which will include adding the names of seven Houston officers who died in the line of duty to the National Law Enforcement Memorial.