HUNTSVILLE, Texas — The man convicted of killing a Houston police officer was executed Wednesday evening, 30 years after the murder.

It was July 19, 1988 when 30-year-old Robert Mitchell Jennings gunned down 24-year-old Elston Howard at an adult bookstore.

Jennings is the first inmate put to death in 2019 both in the U.S. and in Texas. Jennings was one of 224 inmates currently on Texas' death row.

He had been convicted of robbery twice before the 1988 murder and had been on parole for about two months when the shooting occurred. 

Since being paroled, Jennings had gone on a crime spree, committing about 10 robberies, including having already robbed the same bookstore 12 days before Howard's slaying.

Read: Inmate set to be executed for Houston police officer's 1988 murder

On the night of the murder, Officer Howard and his partner were making a routine check of adult bookstores. 

He was issuing a citation to a clerk at Mr. Peeper's Bookstore when Jennings walked in with the intention of robbing the business. 

During the robbery attempt, Jennings fatally shot Howard four times. Jennings was shot in the hand and drove himself to a hospital where he was later arrested. 

A year later, Jennings was sentenced to death. It took the jury about an hour to find him guilty of capital murder. 

Robert Mitchell Jennings hangs his head as he is escorted by two Houston police officers in 1988, hours after he shot and killed Officer Elston Howard.

Jennings' mother was in the courtroom throughout the week-long trial and called it "man's verdict, not God's verdict."

The mother of Robert Mitchell Jennings during an interview with KHOU 11 News in 1989.

"I feel bad for the family. I feel bad for myself. I wasn't there so I don't know what happened. I only know what my son told me and I believe he's innocent." she said.

After his arrest, Jennings confessed to killing Howard, telling police in a tape-recorded statement he was remorseful about what happened and would "face whatever punishment (he had) coming."

Jennings was placed on death row on Nov. 10, 1989, according to the Texas Tribune.

His attorneys have asked the U.S. Supreme Court to delay the execution, arguing Jennings' trial attorneys failed to ask jurors to fully consider evidence — including details of his remorse for the officer's shooting, troubled childhood and possible brain damage — that might have spared him a death sentence.

This undated photo released by Texas Department of Criminal Justice shows death row inmate Robert Jennings.
This undated photo released by Texas Department of Criminal Justice shows death row inmate Robert Jennings.
Texas Department of Criminal Justice

Jennings got an execution stay in 2016. Lower appeals courts have rejected his request to delay Wednesday's execution and the Texas Board of Pardons and Paroles has turned down Jennings' request to commute his sentence. 

Jennings was executed Wednesday at the Texas State Penitentiary in Huntsville also known as Huntsville Unit. It's the oldest Texas state prison and opened in 1849.

The unit houses the State of Texas execution chamber and is the most active execution chamber in the United States.

An updated photo of Robert Mitchell Jennings in court.