LAS VEGAS (Reuters) - A U.S. federal judge in the criminal conspiracy trial of Nevada rancher Cliven Bundy and three other men on Monday warned that prosecutors’ failure to produce documents that could support defense arguments may lead to a mistrial.

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The four men, including two of Bundy’s sons Ammon and Ryan, are accused of conspiring to use the threat of force in a 2014 armed standoff with federal agents near Bundy’s ranch. It galvanized right-wing militia groups challenging U.S. government authority over public lands in the American West.

The revolt was sparked by the court-ordered roundup of Bundy’s cattle by government agents over his refusal to pay fees required to graze the herd on federal land.

Outside the presence of the jury, U.S. District Judge Gloria Navarro on Monday read a lengthy list of documents previously undisclosed by prosecutors that could be used to impeach government witnesses or otherwise bolster defense attorneys’ arguments that their clients felt surrounded by government snipers prior to the standoff. Navarro then sent jurors home until Dec. 20 while the evidence issues are addressed.

Some of the prosecution’s documents were produced so late they violated the federal evidence discovery rules, Navarro said, and as a result could lead her to strike witness testimony, call for a continuance or order a mistrial.

The information would possibly “undermine the confidence in the outcome of the trial,” said Navarro.

“The court is inclined to find that this information was not timely provided,” she added.

Defense attorneys resumed their call for a mistrial.

“The federal government is required to produce documents that assist the defense,” Cliven Bundy’s attorney, Bret Whipple, told Reuters. “The rules dictate fairness, and when the rules are violated the proper remedy is dismissal.”

Navarro ordered a sealed hearing to discuss 14 outstanding evidence issues arising from a series of motions to dismiss the case made by the defense.

Prosecutors previously downplayed the presence of FBI surveillance cameras placed outside the Bundy Ranch near Bunkerville, about 79 miles (120 km) northeast of Las Vegas. The documents include FBI memos that clarified the agency’s support role in the standoff.

One FBI memo noted that an FBI agent carried an AR-15 assault-style weapon, which Navarro said would likely aid defense arguments that the defendants felt endangered by government “snipers.” Navarro also noted the existence of government surveillance logs.

Last week a detention hearing was held for the final four defendants in the standoff case, whose trial has been delayed for nearly one year. That would mark the third group of defendants on trial related to the standoff.

U.S. Magistrate Judge Peggy Leen ordered the conditional release of Cliven Bundy’s sons Dave and Melvin Bundy, Jason Woods and Joseph O’Shaugnessy, who are accused of being “mid-tier” participants in the standoff, according to the indictment. They have been detained for about 21 months.

Six lesser-known participants in the Nevada ranch showdown went on trial as a group earlier this year. Two men were found guilty, one of them sentenced to 68 years in prison. The other is awaiting sentencing.