By John L. Smith

LAS VEGAS (Reuters) - A federal judge on Wednesday declared a mistrial in a criminal case against four people, including Nevada rancher Cliven Bundy, over a 2014 armed standoff with U.S. government agents, and rebuked prosecutors for withholding evidence from the defense.

Bundy, two of his sons and a fourth man were charged with 15 counts of conspiracy, assault and other offenses stemming from the confrontation, which galvanized right-wing militia groups challenging federal authority over vast tracts of public lands in the American West.

U.S. District Judge Gloria Navarro told federal prosecutors they had willfully violated evidence rules in failing to turn over pertinent documents to the defense, adding, "The failure is prejudicial," to ensuring a fair trial.

The ruling came a month after prosecutors began presenting their case to a federal court jury in Las Vegas.

Navarro had warned prosecutors last week she might declare a mistrial after listing documents previously undisclosed by prosecutors that could be used to impugn government witnesses or bolster defendants' arguments that they felt threatened by government "snipers" before the standoff.

In a stern rebuke on Wednesday, Navarro said prosecutors knew or should have known of the existence of memos from Federal Bureau of Investigation agents that might have been helpful to the defense.

Those memos and other documents, about 3,300 pages in all, were not turned over until well after an Oct. 1 deadline, and then only after repeated efforts by Bundy's defense counsel, Navarro said.

Navarro set a retrial date for Feb. 26, 2018, but it is uncertain if there will be a new trial. Acting U.S. Attorney Steven Myhre said prosecutors had yet to decide whether to pursue the case. Defense lawyers will argue for charges to be dismissed at a hearing set for Jan. 8.

"We have a very strong case that this will never be tried again," Bundy's attorney, Bret Whipple, told reporters.

U.S. Attorney General Jeff Sessions weighed in through a Justice Department spokesman in Washington.

"The attorney general takes this issue very seriously and has personally directed that an expert in the department's discovery obligations ... examine the case and advise as to the next steps," said the spokesman, Ian Prior.

UPROAR OVER CATTLE

The April 2014 revolt at the heart of the trial was sparked by a court-ordered roundup of Bundy's cattle by agents of the U.S. Bureau of Land Management (BLM) after Bundy had refused for two decades to pay fees to graze his herds on federal property.

Hundreds of supporters, many of them heavily armed, answered Bundy's public pleas for support and rallied at his ranch near Bunkerville, Nevada, about 75 miles (120 km) northeast of Las Vegas, in a show of force to demand the return of his impounded livestock.

Police and government agents, vastly outgunned, ultimately retreated rather than risk bloodshed, and no shots were fired.

Bundy, his sons Ammon and Ryan, and co-defendant Ryan Payne, a Montana resident prosecutors linked to a militia group called Operation Mutual Aid, have cast the uprising as an act of patriotic civil disobedience against government excess.

"I don’t believe there’s a jury in this country that would convict us," Ammon Bundy said after the hearing, adding, "I believe that God has favored us."

Prosecutors contend Bundy, 71, and his followers were defying the rule of law by threat of violence, rather than engaging in an act of legal protest. The most serious counts the defendants face carry a maximum sentence of 20 years in prison.

Ammon and Ryan Bundy were acquitted last year, along with five people, in a separate conspiracy case over a six-week armed occupation of a U.S. wildlife refuge in Oregon by activists protesting against federal land management.

A would-be fifth defendant in the Nevada case, internet blogger and radio host Peter Santilli Jr., pleaded guilty on Oct. 6 to conspiracy and faces a possible six-year prison term.

Six lesser-known participants in the Nevada showdown went on trial earlier this year. Two were found guilty; one received a prison term of 68 years, a fourth awaits sentencing. Two pleaded guilty to lesser charges in exchange for a maximum one-year jail term.

Yet another group of six defendants from the Bunkerville standoff, including two other Bundy sons, Dave and Mel Bundy, were due to stand trial after the Cliven Bundy case concluded. One of them, Micah McGuire, pleaded guilty in November to conspiracy to impede or injure a federal officer, and faces up to six years in prison when sentenced.

(Reporting by John L. Smith; writing by Peter Szekely in New York and Steve Gorman in Los Angeles; Editing by Steve Orlofsky and Clarence Fernandez)