A man found guilty of the 1988 murder of an officer with the Houston Police Department was put to death on Wednesday.

Along with an accomplice, Robert Jennings entered Mr. Peeper's Bookstore with the intention of robbing the business. He was unaware at the time that Officer Elston Howard, 24, was already inside seeking to arrest the store clerk.

Jennings shot Howard twice in the head, and after Howard fell to the ground, Jennings shot him two more times.

According to Fox News , 61-year-old Robert Jennings was pronounced dead 18 minutes after receiving a lethal injection and some 30 years after fatally shooting Officer Howard at an adult book store.

Outside the prison, more than 100 officers stood vigil. And a motorcycle club that supports police revved their engines, with the roar from the bikes audible in the death chamber, ABC 13 reports.

Houston Police Chief Art Acevedo, asked about the 30-plus years between the crime and the punishment, said he thought "justice delayed is, to an extent, an injustice continued."

"But when the state takes a life, there has to be a process," Acevedo said. "In this case, the day of reckoning is here. It's a solemn occasion. For us it's a celebration of a life well-lived by Officer Howard. We're a family. That's why we're here."