WASHINGTON (AP) — Attorney General Eric Holder expressed regret Tuesday that the Justice Department sent Congress a misleading letter early this year rejecting allegations of serious problems in the arms trafficking investigation called Operation Fast and Furious. At a Senate Judiciary Committee hearing, the attorney general said that the probe was flawed in concept as well as in execution, never should have happened and "it must never happen again." Holder's comments came as the panel's ranking Republican, Sen. Charles Grassley of Iowa, said the operation represented an "utter failure" by federal law enforcement officials to enforce existing gun laws.