This image from video provided by WJLA-TV shows Ryan Loskarn, chief of staff to Sen. Lamar Alexander, R-Tenn., being escorted from his Washington home by U.S. Postal Inspector police on Wednesday, Dec. 11, 2013. Alexander said he has placed Loskarn on unpaid leave after learning the staffer's home was being searched as part of a child pornography investigation. (AP Photo/WJLA-TV)

NASHVILLE, Tenn. - The fired chief of staff for U.S. Sen. Lamar Alexander was expected Thursday 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."

Several messages left for Loskarn were not immediately returned.

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 expected to be scheduled Thursday 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 that it will be up to the courts to judge Loskarn, but "under these circumstances, he cannot continue to fulfil his duties as chief of staff."

Alexander is a former Tennessee governor who also has served as the U.S. Secretary of Education. He was first elected to the Senate in 2002. He said his office is fully co-operating with the investigation.

According to Loskarn's biography on his LinkedIn account and in previous communications for Alexander's office, Loskarn graduated from Tulane University in 2000 and went to work for retired Republican U.S. Rep. Wally Herger of California as a legal assistant.

He later was the communications director for U.S. Rep. Marsha Blackburn, a Tennessee Republican, and eventually became staff director for the Senate Republican Conference.

Loskarn had served as Alexander's chief of staff for two years.

News from © The Associated Press, 2013