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Authorities were continuing to hunt for a suspect Saturday in the "execution-style" shooting of a uniformed Texas sheriff's deputy, while warning that "dangerous rhetoric" against law enforcement has "gotten out of control."

"We've heard black lives matter — all lives matter," a visibly angry Harris County Sheriff Ron Hickman told reporters. "Well, cops' lives matter, too."

A person of interest was voluntarily taken in for questioning early Saturday, and authorities said that he was cooperating but did not provide details about his relationship to the case.

The emotional news conference came after Harris County Sheriff’s Deputy Darren H. Goforth, 47, was ambushed Friday night after exiting the convenience store of a Houston Chevron gas station.

Goforth had just filled his patrol car when a man "walked up behind him and literally shot him to death," Hickman said at a news conference overnight.

After Goforth fell to the ground, the suspect stood over him and fired several more times.

"It appears to be an unprovoked execution-style killing of a police officer," Hickman said.

But officials continued to plead for help from the public. Hickman said no one had been taken into custody, and asked for any witnesses to the shooting of the loving "father, husband, son and committed law enforcement officer" to come forward.

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Surveillance video of the suspect and his vehicle, from Harris County Sheriff's Office Handout / Harris County Sheriff's Office

Earlier, police described the suspect as a "dark-complected male" in his early 20s wearing a white T-shirt and red shorts. He was reportedly driving a dark red or maroon extended cab Ford Ranger.

This was an unprovoked attack. Deputy Goforth left behind a wife and two children. Please continue to pray during this difficult time. — HCSOTexas (@HCSOTexas) August 29, 2015

"We will hunt this person down and do our best to bring that person to justice," Hickman said.

Investigators do not know what the motive was and believe the suspect targeted Goforth simply because he was in uniform.

"The motive for this is absolute madness," Hickman said.

Deputy Darren Goforth, pictured in this official image from Harris County Sheriff's Office. Handout / Harris County Sheriff's Office

Goforth, a 10-year veteran of the Harris County Sheriff’s Office, stopped for gas around 9:30 p.m. ET after having just finished responding to a routine call about a car accident. Hickman said there was no indication that call had anything to do with the killing.

Harris County District Attorney Devon Anderson said Saturday that law enforcement needs support despite there being a "few bad apples" — an apparent reference to the recent spate of police shootings of unarmed people.

"There are a few bad apples in every profession. That does not mean there should be open warfare declared on law enforcement," she said. "The vast majority are here to do the right thing. They care about their community. What happened last night is an assault on the fabric of society."

Horrific news from my hometown. My prayers are with the family and community. Help find the suspect: http://t.co/TixFfn7D1e — Ted Cruz (@tedcruz) August 29, 2015

Hickman added that it was "time to drop the qualifier" in the rally cry, "Black lives matter," which has proliferated online following the police shootings of Michael Brown and other young black men.

Police are reviewing surveillance footage from the gas station. The FBI, Texas Rangers and U.S. marshals are assisting in the search for the suspect, Hickman said.

The deputy had a wife and two children.

We will not rest until this suspect has been located and justice is served to the family of Deputy Darren Goforth. — Ron Hickman (@SheriffHickman) August 29, 2015

"Tonight we lost one of our brothers. This is an extremely difficult time right now for the Harris County Sheriff's Office," the department wrote on its Facebook page. "We ask that you continue to give us your prayers and thoughts. Continue praying for this deputy's family."

Houston Mayor Annise Parker ordered city flags lowered after his killing.

On Saturday morning, balloons, flowers and candles were placed at the Chevron station gas pump that Goforth used before he was killed. A note at the impromptu memorial read, "Gone but never forgotten R.I.P Deputy Goforth," The Associated Press reported.

Hickman tweeted: "We will not rest until this suspect has been located and justice is served to the family of Deputy Darren Goforth."