U.S. District Judge Larry Alan Burns appointed Dr. Matthew Carroll to perform Loughner's mental-competency exam, according to the Associated Press.
The AP says the decision to appoint Carroll to Loughner was "in deference to the defense's concern about the impartiality of Bureau of Prisons staff."
Carroll's other high-profile appointment was to double-murderer and sex offender John Albert Gardner III -- who was sentenced last year to two life sentences without the possibility of parole for the rape and murder of two teenage girls in California.
Lougher is accused of the January 8 shooting rampage outside a Tucson Safeway grocery store that left six dead, including federal Judge John Roll, and wounded 13 others, including Congresswoman Gabrielle Giffords.
The U.S. Attorney General's Office announced a 49-count federal indictment against Loughner in March based on the attempted murders of Giffords and her two aides, the murders of Roll and Giffords' aide Gabriel Zimmerman, and four counts of causing the death of participants at a federally provided activity.
Loughner's competency hearing is scheduled for May 25.
