Updated

Houston police stepped up the search Friday for a gunman who they say ambushed and seriously wounded a Texas deputy constable, after investigators concluded that a suspect they had questioned was not involved in the shooting.

Crime Stoppers of Houston, a nonprofit public safety organization, doubled to $10,000 a reward for information leading to the arrest of the person responsible for the shooting of Harris County Deputy Constable Alden Clopton.

Authorities said Clopton, 48, was shot four times from behind late Wednesday as the assailant stood on the other side of a four-lane road in a neighborhood about three miles south of downtown Houston. Clopton was trying to assist another officer in a routine traffic stop. Police are unsure of the motive but said they believe it was an ambush.

Investigators initially focused on questioning a man with a long criminal history, Chester Irving, 46, who appeared at a fire station near the scene about 30 minutes after the shooting. According to Harris County court records, the Houston resident had been arrested 32 arrests beginning in 1988 for a variety of offenses involving burglary, drugs, prostitution, forgery and weapons violations.

"He showed up with a gun magazine, which you slide into the gun," Houston police spokesman Keese Smith said. Authorities had said Thursday that police were not actively looking for anyone else.

On Friday they reversed course and said that Irving had not been involved in the shooting and instead charged him with illegally possessing a gun. As a felon, Irving was not allowed to have a gun.

"During the course of the interviews we were doing with him he said, 'Yeah, I have a gun but I hid it before I came to the fire station,'" Smith said.

Police gave no explanation of why they had ruled out Irving.

"It's fair to say we're still looking for the shooter," Smith said.

Doctors have said Clopton suffered wounds to his chest and abdomen and should fully recover, but will remain in the hospital for about a week.

Authorities initially said that Clopton's life might have been saved by wearing a bulletproof vest. But the Harris County Precinct 7 Constable's Office said Friday it was not clear if the vest was removed by paramedics at the scene or if Clopton was not wearing it when the shooting happened.

"We will not be able to give a definite answer until the investigation has been completed," Constable's Office spokeswoman Pamela Greenwood said.

Constables in some regions are tasked with serving warrants and providing court security, but in the Houston area they generally provide the same policing coverage as other law agencies.

Clopton is an 11-year veteran of the Harris County Precinct 7 Constable force and comes from a law enforcement family. He has three brothers who are law officers, his wife is a Harris County sheriff's deputy and a son is an officer in Biloxi, Mississippi.

Clopton is the second Harris County law officer to be shot from behind in an unprovoked attack in the past year. Texas prosecutors in August charged a 30-year-old man with capital murder in the killing of sheriff's Deputy Darren Goforth, who was gunned down while filling his patrol car with gas in what officials described as a "senseless and cowardly act."