HCSO said Coddou was assisting a crash scene when a passing vehicle fatally hit him on the highway on the county's northwest side. He was 50 years old.
SEE MORE: Who was John Coddou, the Harris County deputy killed in a crash on Grand Parkway?
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The Houston Police Department was among the first agencies to send condolences to their grief-stricken county counterparts.
"We extend our deepest condolences to our (HCSO) family and the family of Deputy Coddou, who tragically lost his life in the line of duty while assisting a motorist this morning. Please keep the deputy's family and the HCSO community in your prayers," HPD posted on X, formerly known as Twitter.
The Texas Department of Public Safety, still reeling after an alleged stolen big rig slammed into its Brenham driver's license office, extended its thoughts and prayers to Coddou's family and loved ones.
SEE LATEST UPDATE: Brenham DPS crash involving allegedly stolen big rig claims 2nd life weeks later, officials say
"We extend our deepest condolences to our local partner (HCSO) as they mourn the loss of a fellow deputy," DPS posted through its Southeast Texas Region account.
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The FBI's Houston Field Office responded to an HCSO tweet as Sheriff Ed Gonzalez addressed the grim news.
"We are so sorry for your loss," FBI Houston tweeted.
"END OF WATCH," Harris County Precinct 4 Constable Mark Herman's office began in their tweet. "(May) God grant him eternal peace."
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"This is a tragedy," the Houston Police Officers' Union added. "Our thoughts and prayers are with the family, friends, and coworkers of Deputy Investigator John Coddou. Rest in peace, brother."
Even agencies outside Harris County, including Edinburg and Pflugerville ISD police departments, supported the sheriff's office.
"Rest easy brother, we have it from here," Pflugerville ISD police tweeted.
While HCSO didn't speak at length about the deputy's family, the nonprofit 100 Club, a decades-old organization supporting dependents of officers and firefighters killed or injured in the line of duty, said Caddou is survived by his wife, whom the group will present with a $20,000 check to help with any immediate financial needs.
The group also appealed to the public for contributions to the 100 Club Survivor's Fund.