WACO, Texas — No one will be convicted or otherwise held accountable for the 2015 shootout between rival biker gangs in Waco restaurant parking lot that left nine people dead and at least 20 injured, prosecutors in Central Texas said Tuesday.

In a statement announcing all charges will be dropped in the deadliest biker shooting in U.S. history, McLennan County District Attorney Barry Johnson said any further effort to prosecute the case would be a "waste of time, effort and resources."

"In my opinion, had this action been taken in a timely manner, it would have, and should have, resulted in numerous convictions and prison sentences against many of those who participated in the Twin Peaks brawl," Johnson said. "Over the next three years the prior district attorney failed to take that action, for reasons that I do not know to this day."

A McLennan County deputy stands guard near a group of bikers in the parking lot of a Twin Peaks restaurant Sunday, May 17, 2015, in Waco, Texas. Rod Aydelotte / AP file

The shooting outside a Twin Peaks restaurant in Waco on May 17, 2015, involved rival biker gangs, the Bandidos and Cossacks, and occurred as bikers from various groups were gathering to talk over matters of concern. Fights and gunfire broke out. Waco police officers monitoring the gathering also fired on the bikers, killing at least two.

Surveillance footage showed many bikers running from the scene and ducking for cover after gunshots rang out. A smaller number could be seen pointing and firing weapons, slinging a chain or participating in fistfights. Law enforcement officers recovered dozens of firearms, knives and other weapons from the restaurant and adjacent parking lot, many of which officers organized indiscriminately into piles on the pavement and in the back of a police vehicle, dash-cam video showed.

Law enforcement officials took the extraordinary step of arresting 177 bikers after the shooting, then charged 155 of them with engaging in organized criminal activity. Many were held on a $1 million bond.

Former District Attorney Abel Reyna ultimately dropped charges against all but 24 and re-indicted them on riot charges. Those were the cases that came to an end Tuesday.

Only one case was prosecuted in court and that ended in a mistrial.

More than 100 bikers have filed civil rights lawsuits alleging McLennan County, the city and others violated the plaintiffs' civil rights by arresting them without probable cause after the shooting.

"It's a travesty that so many people were rounded up and then investigated, instead of vice versa," Mark Snodgrass, president of the Texas Criminal Defense Lawyers Association, said Tuesday. "A lot of these people's lives were put on hold for four years."

In a statement, Reyna said he disagrees "with the overall result as well as several statements and accusations within Mr. Johnson's press release; however, it is solely his decision on how to proceed with any case in the District Attorney's Office."