WASHINGTON (AP) — Prosecutors say a witness in the criminal case against four former Blackwater Worldwide security guards gave misleading testimony to a grand jury investigating the 2007 Baghdad shooting that killed 14 Iraqi civilians and injured at least 18 others.

In response to the disclosure, U.S. District Judge Royce Lamberth has demanded a written explanation of why the U.S. attorney’s office handling the case did not immediately notify lawyers for the Blackwater guards facing trial of evidence that could be useful in their defense.