WASHINGTON - A sheriff's deputy was gunned down from behind and shot multiple times at close range while fueling his patrol car in a "cold-blooded assassination," Texas authorities said Saturday, appealing for help in tracking down the killer.

The fatal shooting prompted an emotional reaction from local officials, who pointed their finger at ramped-up rhetoric against police officers in the United States in protests against perceived police brutality, although they had yet to identify a suspect.

A gunman came up from behind and opened fire on Darren Goforth, 47, in a Houston area gas station Friday night, police said.

"The deputy then fell to the ground and the suspect then continued over to him and shot the deputy again multiple times as he laid on the ground," said deputy Thomas Gilliland on a local ABC affiliate. Sheriff Ron Hickman said the killing was one of the worst he has seen.

"I've been in law enforcement 45 years and I don't recall another incident this cold-blooded and cowardly," Hickman said.

Police are looking for a man with a dark complexion seen on gas station video footage and they released photos of him and his car on Twitter.

Goforth, a white, 10-year veteran of the force, was married with two children. "Our hearts are broken tonight," tweeted the sheriff's office. "I plead today for the public's assistance in our efforts to bring the suspect to justice he so rightfully deserves," Hickman added in an update, saying the suspect was still at large.

He said the "working motive" for investigators was "absolute madness."

The shooting has no clear motive, according to police, but by claiming the shooting was part of a dangerous rhetoric in America the local officials connected the killing to a series of protests in major American cities against the police.

"At any point when the rhetoric ramps up to the point where calculated, cold-blooded assassination of police officers happen, this rhetoric has gotten out of control," Hickman said.

"We've heard black lives matter. All lives matter. Well, cops' lives matter, too. So why don't we just drop the qualifier and say lives matter."

"Black Lives Matter" became the recent rallying cry for protesters demonstrating against a string of killings of unarmed black men by police officers.

Starting with the gunning down of 18-year-old Michael Brown in Missouri one year ago, demonstrations around the United States have condemned law enforcement's use of deadly force and called for reform of the nation's police departments.

The protests that kicked off in Ferguson renewed a debate on race and policing in America and led to reform efforts leading all the way to the White House.

But the focus on condemning cops also caused police forces to say they were being unfairly targeted, increasing their risk in an already dangerous field.

Two police officers were gunned down from behind in New York last year by a man who was seemingly angered by recent police killings.

New York police lashed out at government officials they thought were being overly sympathetic to a protest movement that had dangerous implications.

Statements from Texas on Saturday lined up with that feeling of neglect and anger.

"It is time for the silent majority in this country to support law enforcement," said district attorney Devon Anderson.

"There are a few bad apples in every profession, that does not mean that there should be open warfare declared on law enforcement," she said.