WASHINGTON — Donna R. Andrieu, an assistant district attorney in New Orleans, had the unenviable task at the Supreme Court on Tuesday of defending her office’s conduct in withholding evidence from a criminal defendant. She made the least of it.

Her halting and unfocused presentation elicited one incredulous question after another. The argument culminated in back-to-back rebukes from Justices Elena Kagan and Sonia Sotomayor.

Justice Kagan said she could not understand why the Orleans Parish District Attorney’s Office persisted in defending its conduct. “Did your office ever consider just confessing error in this case?” she asked.

Justice Sotomayor made a broader point about the office, which has repeatedly been found to have violated Brady v. Maryland, the 1963 Supreme Court decision that requires prosecutors to turn over favorable evidence to the defense.