The three men gathered in a Los Angeles warehouse, bringing a 12-gauge shotgun, a .38 revolver, zip ties for handcuffs and a duffel bag to carry the 20 to 25 kilograms of cocaine, worth more than $500,000 wholesale, they expected to steal.

The men had criminal records, were broke and dazzled by their imminent wealth. They met with a drug courier who had offered to help them rip off his suppliers. Those guarding the cocaine shipment would be armed, he had warned, so come ready for gunplay.

As the crew made final preparations, federal agents pounced. The stash house and the cocaine were imaginary, and the "courier" an undercover agent of the Bureau of Alcohol, Tobacco, Firearms and Explosives. Though the drugs were fictitious, the three were charged with conspiracy to distribute more than 5 kilograms of cocaine — which carries a mandatory minimum sentence of 10 years — and faced an additional mandatory five years for bringing guns.

"Stash-house stings" like this one in 2013 have sent well over 1,000 of the country’s most "violent, hardened criminals" to prison, according to the bureau, which has made a specialty of the ruses. The agency says it has conducted about 365 of these stings over the last decade, removing from the streets career criminals who are "willing to kill and be killed," sometimes for multidecade terms, with less risk to agents and neighbors than raids on real stash houses.

But earlier this year, the judge in this Los Angeles case dismissed the charges against two of the defendants on the rarely invoked grounds of "outrageous government conduct." Judge Otis D. Wright II of U.S. District Court described the bureau in his March decision as "trawling for crooks in seedy, poverty-ridden areas — all without an iota of suspicion that any particular person has committed similar conduct in the past."

Similar prosecutions have nearly always held up in court, and the agency strongly defends its methods and choice of targets. But over the last year a growing number of federal judges have questioned the tactic.

A second judge in Los Angeles dismissed similar charges in May. The federal appeals court in Chicago last week mandated a new trial to allow evidence of possible entrapment. Other judges have demanded data from the bureau to help them explore whether the stings, which nearly always land black or Hispanic defendants, involve illegal racial targeting.

The stash-house stings are a prime example of the spreading federal use of undercover agents in many fields, a trend that law enforcement officials say is efficient and safe but is raising unease among civil liberties advocates.

On Thursday, a federal appeals court in California heard the government’s motion to reinstate criminal charges in the case that Wright criticized, and its call for the judge’s removal from the case for bias.

"The conspiracy was real; the guns were real; the defendants’ intent to use them to violently rob a cocaine stash house was real; and the defendants’ criminal histories were real," the federal prosecutors argued in their brief. The supposed stock of cocaine had to be set high, they said, to make the proposal credible.

In May, also in Los Angeles, Judge Manuel L. Real of U.S. District Court dismissed charges against three other men, saying the government "steers too close to tyranny." He said that the agents initially knew little about the defendants except that "they were from a poor neighborhood and minorities." The government has appealed.

And last week, in another setback for federal agents, the 7th U.S. Circuit Court of Appeals mandated a new trial for a defendant in a Chicago suburb who claimed he had been pressured into planning the robbery for which he was sentenced to nearly 27 years. The trial judge had erred, the circuit court ruled, by not allowing an entrapment defense.

In a separate line of attack on the drug stings, defendants in Chicago and elsewhere have filed motions to require the bureau to provide data on the racial makeup of sting targets, and information on how the agency selects its targets.

In one case, the agency asked the court to dismiss charges rather than be required to comply. In several others, after judges found at least suggestive evidence of racial targeting and approved the data request, the agency has complied, though the information remains under seal.

The goal, defense lawyers say, is to build a case that the bureau engages in racial profiling and selective prosecution, which could result in the dismissal of charges.

Defense lawyers in the Chicago cases calculated that the 25 known stash-house stings in that judicial district since 2006 involved 75 black, 16 Latino and six white defendants. The 13 cases since 2010 involved 45 black defendants, 14 Latinos and one white person.

An unofficial national review of court files by USA Today, published in July, found that 9 in 10 of those imprisoned through such stings were black or Hispanic, well above the share of minorities convicted on charges of other federal robbery, gun and drug offenses.

Ginger L. Colbrun, a spokeswoman for the bureau, strongly denied racial targeting. The agency, she said, works with local partners "to identify the areas and people that are most violent in a community."

Entrapment is difficult to prove when defendants show a willingness to participate, and the efforts to prove racial discrimination may also be hard to sustain in court.

But that may not be the important point, some critics say.

"Is this where we want to put our law enforcement resources — giving people the opportunity to commit a crime and then putting them in prison for decades?" asked Katharine Tinto, a professor at the Benjamin N. Cardozo School of Law in New York.

Colbrun, the bureau spokeswoman, said: "Targets of these investigations are only pursued if they show a predisposition to commit these acts and a propensity for violence."

But one criticism of the stings is that they sweep up, along with career offenders, small-time criminals with no history of armed robbery or serious drug dealing.

At least one defendant in the Los Angeles case that Wright dismissed did not appear to be a predictable stash-house robber.

According to prosecutors, the operation was initiated when Cedric Marquet Hudson, who had served time for crack dealing and gun offenses and had gang affiliations, told a paid informant that he was looking for a robbery opportunity.

The informant put Hudson in touch with an undercover bureau agent. Hudson brought along Joseph Cornell Whitfield, whom prosecutors describe as a fellow gang member with a similar history of drug and gun crimes. (Both were on parole and wearing ankle bracelets.)

At the repeated urging of the fake courier, as defense lawyers have argued in the circuit court case, they bolstered their crew by bringing in Antuan Duane Dunlap, an acquaintance of Whitfield’s who was then homeless. He boasted of past robberies but had a less extensive criminal history than the other two.

Dunlap’s lawyer, Lawrence J. Litman, said in an interview that "my client got picked up off the street, dragged in at the last minute," and that the agents did not even know his identity until the arrest.

Hudson and Whitfield had already pleaded guilty when Wright dismissed the charges against Dunlap. All three remain in jail, resolution of their cases delayed while they await the appeals court ruling.

In a divided decision in May, the same court, the 9th U.S. Circuit Court of Appeals, declined to endorse the claim of outrageous government conduct in another, similar sting.

But if the appeals court does restore the criminal charges against Dunlap, Litman said, "we’ll be looking at the racial issue next."

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