Prosecutors in Washington are building a new criminal case against a group of former Blackwater private security guards whom the government believes wrongly killed more than dozen Iraqi civilians in a Baghdad shootout in 2007...Assistant U.S. attorney...Anthony Asuncion...said...that the government is moving forward to obtain a superseding indictment. He did not discuss potential new charges...Steptoe & Johnson partner Brian Heberlig, a lawyer for Paul Slough, said in court he anticipates challenging any new indictment on the ground that witnesses could have been exposed to information that was supposed to remain confidential.