"Andrea Foulkes is an outstanding prosecutor with an impeccable record for integrity, professionalism, and dedication to public service," First Assistant U.S. Attorney Louis Lappen said in a statement.
Federal courts had previously found the work of Williams' now-disbarred trial lawyer "unconstitutionally deficient," although they stopped short of throwing out the death sentence.
Federal public defenders who work solely on death-penalty cases have represented Williams since 1996. They say the errors found in his case are not uncommon. Pennsylvania has about 200 people on death row.
"That we were talking about executing somebody who meets his lawyer a day before trial is an indictment of the system. The fact we're talking about this new evidence a week before execution is a bigger indictment of the system," public defender Victor Abreu said.
Williams still has a clemency petition pending with the state Board of Pardons, which reopened his case Thursday but did not rule.
"He's hanging in there. It's not an easy situation," said Shawn Nolan, another public defender.