The federal judge presiding over the landmark case against the New York police department's controversial stop-and-frisk policy has complained that witnesses have perjured themselves during the hearings.

Judge Shira Scheindlin ordered two officers to come back to court this week, after expressing concerns about their earlier testimony. In heated exchanges with the officers' lawyers, she alleged that other witnesses had committed perjury.

The two officers returned to court on Monday, in order to be seen by Leroy Downs, an African American resident of Staten Island who claims they violated his rights in a 2008 stop. The officers had first testified last week. One said he could not recall the stop, while the other said it did not happen. Downs' detailed description of the alleged encounter, as well as earlier complaints about the incident that were upheld, has called the officers' testimony into question and underscored ongoing challenges in the landmark trial.

The drama began last week, as officers James Mahoney and Scott Giacona took the witness stand. The pair were questioned about a stop alleged to have taken place on 20 August 2008, while the men were part of a plainclothes Brooklyn gang unit temporarily assigned to work on Staten Island. The officers were accused of stopping, frisking and searching Downs outside his home. Downs filed a complaint about the incident with the Civilian Complaint Review Board (CCRB), an institution tasked with investigating allegations of police abuse. In January 2009, the officers testified under oath about the stop in an interview with the CCRB.

Both men told the CCRB that they did not recall stopping and frisking Downs. The CCRB ultimately upheld a complaint against both officers for failing to provide their badge numbers during the encounter, after being asked to do so.

In testimony last week, Mahoney repeated his claim that he did not recall the stop. Giacona, meanwhile, was shown a photograph of Downs on the day the stop happened. When asked if he recognized the man or had ever stopped him, Giacona said: "No."

The two officers and Downs had been scheduled to testify on the same day, which would have placed all three in the same room at the same time, but due to a reordering of the witness list the police officers testified earlier in the week and Downs took the stand on Friday. He described the stop in detail, and said that he later recognized the two officers when he went to file a complaint at the 120th precinct.

Scheindlin was visibly disturbed by the discrepancies between Downs' account and the that of the officers. "It's kind of an important point since they swore under oath that this never happened," she said. "If the court concludes that any witness has perjured him or herself, that's a serious problem." She ordered the officers to appear again on Monday. "I'm directing the city: reproduce these two officers. Call their precincts. Get them in here," Scheindlin said.

Scheindlin then indicated that she believes she has already heard perjured testimony in the trial, which is now entering its sixth week.

"Do I think I've heard some perjured testimony in the last four weeks? From somebody? From all the witnesses I've heard?" Judge Scheindlin asked. "Yeah, I think somebody, of all those four weeks, I think I've heard it already. Proving it is a different issue. You don't know who I'm referring to. It could be a plaintiff witness. It could be a defense witness. I think I've already heard it."

On Monday morning, Mahoney, Giacona and Downs all returned to court. The officers denied having ever seen Downs before, while Downs insisted they were the officers who had stopped him. "Those were the two," he said. The episode raises an ongoing tension in the historic stop-and-frisk trial: how to assess the testimony provided.

Andrew Quinn of the Sergeants' Benevolent Association, representing the officers on Monday, said: "It's not unusual for officers in a gang unit to have thousands of interactions," arguing that his clients could not be expected to remember every single street-level encounter.

Darius Charney, an attorney with the Center for Constitutional Rights, which is representing the plaintiffs, said: "While for an officer it may be no big deal because they've done hundreds of thousands of stops over the years and one stop isn't a big deal to them, that stop is a very big deal to the people who are stopped.

"To hear Mr Downs' detailed description of what happened to him, his efforts to file a complaint and find out who these officers are, to suggest that he's just making it all up is outrageous, and I think that's what the judge was reacting to." © Guardian News and Media 2013