Mr. Johnson, recently elected to the post, cited what he described as a real possibility that conviction on even those lesser charges would be overturned on appeal. He also said that statutes of limitations made it impossible to pursue charges against anyone else.

And he expressed frustration with the course of the case, saying that there could have been real accountability for the shootout if his predecessor, Abel Reyna, had done a better job. But he said Mr. Reyna had failed to focus on the people most responsible for the carnage and bring them to trial with charges tailored to fit their individual culpability.

Mr. Johnson said Mr. Reyna should have used the available time and resources “to charge only those offenses where the admissible evidence would support a verdict of guilt beyond a reasonable doubt.” And he added, “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.”

Only one defendant was ever tried: Jacob Carrizal, the president of the Bandidos chapter in Dallas. The case was declared a mistrial in 2017 after a majority of jurors voted to acquit him.

Mr. Johnson defeated Mr. Reyna in a Republican primary last year after sharply criticizing Mr. Reyna for the conduct of the shootout prosecution.