NASHVILLE (AP) — The fired chief of staff for U.S. Sen. Lamar Alexander is due to appear in federal court after the aide’s arrest on probable cause for possession and distribution of child pornography, authorities said.Ryan Loskarn was a rising star who had spent the past decade working his way up to increasingly important posts in the U.S. House and then the Senate.Alexander, R-Tenn., said in a news release Wednesday he was “stunned, surprised and disappointed” by the allegations against his newly departed staffer. U.S. Rep. Marsha Blackburn, another Tennessee Republican for whom Loskarn had worked as communications director, said she was “shocked, saddened, and stunned.”Court documents surrounding Loskarn’s arrest were under seal Wednesday, so little is known beyond a short statement from U.S. Department of Justice spokesman Peter Carr.Carr said Loskarn was arrested based on probable cause for possession and distribution of child pornography charges and remained in custody pending a court hearing in federal court in Washington.Video taken by WJLA-TV showed Loskarn being led Wednesday from his southeast D.C. home by U.S. Postal Inspector police, who put him in a black SUV. Loskarn appeared to be in handcuffs, although a coat was draped over his arms. A spokeswoman for the agency said she could not comment on the arrest.Alexander initially placed Loskarn on unpaid leave Wednesday morning, but before the end of the day he had replaced Loskarn with David Cleary, the senator’s legislative director.Alexander said in a statement it will be up to the courts to judge Loskarn, but “under these circumstances, he cannot continue to fulfill his duties as chief of staff.”Alexander is a former Tennessee governor who also has served as the U.S. Secretary of Education.