Attorney General Jeff Sessions on Thursday ordered an investigation into the high-profile prosecution of anti-government ranchers in Nevada, after a judge declared a mistrial, saying that prosecutors had improperly withheld evidence from the defense.

The case against Cliven D. Bundy, his sons Ammon E. Bundy and Ryan C. Bundy, and a supporter, Ryan W. Payne, stemmed from an armed standoff in 2014 between federal agents on one side, and the Bundys and people who flocked to their cause on the other.

On Wednesday, in Federal District Court in Las Vegas, Judge Gloria M. Navarro charged that prosecutors had failed to disclose material they were required to turn over to the defense, including video taken surreptitiously within the Bundy ranch during the standoff, and evidence that F.B.I. agents were involved in the incident. It is unclear whether the case will go to a second trial.