Deputy Darren H. Goforth died Friday night when a gunman came up behind him and shot him in the back as he returned to his patrol car after fueling it near Houston.
When he fell, the shooter stood over him and fired some more, authorities said.
Goforth was in his Harris County Sheriff's Office uniform at the time of the shooting. Officials said it was an "execution-style" attack.
As a heartbroken community mourned the officer, his wife, Kathleen Goforth, paid tribute to the man she called her best friend.
"My husband was an incredibly, intricate blend of toughness and gentility," she told CNN affiliate KTRK
Goforth was guided by an inner moral compass, she said, adding that she was lucky to have been married to him.
"He was who you wanted for a friend, a colleague and a neighbor," his wife said.
A suspect identified as Shannon J. Miles has been in police custody since Saturday and faces a capital murder charge.
His criminal history includes resisting arrest, trespassing, evading detention and disorderly conduct with a firearm, Sheriff Ron Hickman said.
Though the motive of the shooting is unclear, there's no indication so far that the suspect and deputy knew each another, authorities said.
Goforth was targeted simply because "he was wearing a uniform," Hickman said.
"This is the kind of thing that drives you right down to your soul," he said. "It strikes at the heart of who we are as peace officers. ...This was just a cold-blooded execution."
About 30 minutes before the shooting, Goforth had investigated an accident, but Hickman said it's unclear whether there was a connection to the attack.
The suspect shot Goforth, 47, while he was filling up his patrol car at the gas station, Hickman said.
"Deputy Goforth was refueling his vehicle and returning to his car from inside the convenience store when, unprovoked, a man walked up behind him and literally shot him to death," he said.
The 10-year veteran of the Harris County Sheriff's Office died at the scene.
"I have been in law enforcement (for) 45 years," the sheriff said. "I don't recall another incident this cold-blooded and cowardly."
'Rhetoric ... out of control'
The victim is white while the suspect is African-American.
Authorities appeared to link Goforth's killing to the anti-police tensions that have spread nationwide over the deaths of unarmed African-American men at the hands of white officers.
Hickman and Harris County District Attorney Devon Anderson addressed the nationwide debate over the relationship between police officers and the public. The sheriff referred to it as "dangerous national rhetoric."
"There are a few bad apples in every profession," Anderson said. "That does not mean there should be open warfare declared on law enforcement."
Hickman warned that the tension against officers is getting out of hand.
"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 just say 'Lives matter,' and take that to the bank."
In addition to his wife, Goforth leaves behind two children, ages 5 and 12.
A Houston nonprofit that supports families of officers and firefighters who die on duty said the group will give $20,000 to Goforth's wife and children.
The attack Friday adds to a grim tally. With that included, 23 law enforcement officers have been shot to death so far this year nationwide, according to the Officer Down Memorial Page
Traffic incidents are the leading cause of officer fatalities in the U.S., followed by shootings.