A federal jury in the District this week acquitted two men and was unable to reach a verdict against three others on charges that they tried to bribe foreign officials to win business — the latest courtroom setback for the Justice Department in its largest sting operation targeting such illegal payments by U.S. citizens.

The sting, which involved a fictitious deal to outfit the presidential guard of a small African country and a $1.5 million bribe to that nation’s defense minister, was lauded in early 2010 by Justice Department officials after FBI agents from the bureau’s Washington field office arrested 22 men and women and charged them with violating the Foreign Corrupt Practices Act (FCPA). But the case has hardly been a smashing success in the courtroom.

The first trial, of four defendants, ended in a mistrial last year after jurors were unable to reach a verdict. The Justice Department said it will retry the men. The second trial, of six others, lasted 15 weeks — and also has yielded little for the prosecution.

U.S. District Judge Richard J. Leon threw out conspiracy charges against all six, acquitting Stephen Giordanella, 52, of Parkland, Fla., before the case reached the jury. On Monday, after deliberating for two weeks, the jury acquitted R. Patrick Caldwell, 63, a retired Secret Service agent who lives in Fredericksburg, and John Godsey, 39, of Atlanta, of the remaining charges against them.

The jury could not reach a verdict against three others — Mark Morales, 39, of Apollo, Fla., and siblings John Mushriqui, 30, and Jeana Mushriqui, 32, of Upper Darby, Pa. — forcing Leon to declare a mistrial Tuesday.

Justice Department officials did not say whether they would retry Morales or the Mushriquis. They also did not indicate whether they planned to try the remaining 13 defendants. “Given the pending nature of related cases, we would decline to comment further at this time,” Justice Department spokeswoman Laura Sweeney said.

After leaving the courtroom on Monday, Caldwell said he was pleased by the verdict and echoed the words of Raymond Donovan, who was a top Reagan administration official, after he was acquitted of corruption charges.

“I have been thinking a lot about this, and I wonder what office I need to visit to get my reputation back,” said Caldwell, who retired from the Secret Service in 2004 before joining private industry. James L. Jones, a former national security adviser, testified on his behalf.

In both trials, defense lawyers savaged what they called sloppy work by the FBI during the 2009 sting. They repeatedly criticized undercover agents and their key informant for describing the illicit payments vaguely and never uttering the words “bribe” or “kickback.” Agents and the informant called the payments a “commission,” a phrase the defendants’ attorneys argued they either didn’t notice, didn’t understand or didn’t hear. (Caldwell is hard of hearing.)

“You are not supposed to dance around and have little words that kind of slide in and out and talk about commissions rather than something clearer,” Mike Madigan, Godsey’s attorney, told jurors in closing arguments. “That’s not the way an honest sting operation is supposed to work.”

Defense attorneys also hammered the informant, Richard Bistrong, a former law enforcement equipment industry executive, saying that he could not be trusted because of his checkered past.

Bistrong — a drug addict who frequently had sex with prostitutes and routinely bribed foreign officials to win contracts — began cooperating with the FBI in 2007 shortly after his own company caught him violating the FCPA. They told jurors that Bistrong exaggerated and lied in the hopes of winning a reduced sentence.

“You simply cannot have faith in the government’s case,” Steven McCool, Morales’s attorney, told jurors, adding that it is “riddled with reasonable doubt.”

Federal prosecutors countered that executives in the arms industry never would have used words such as “bribe” or “kickback” in such a deal, and that FBI agents wanted the sting to appear realistic. They also said that the context of the recordings makes it clear that the defendants knew what was going on.

While acknowledging Bistrong’s flaws, prosecutors urged jurors to trust him, saying that tapes and other evidence corroborated his testimony.