Ryan Loskarn, Tennessee Senator Lamar Alexander's chief of staff, has been placed on unpaid leave as part of a child pornography investigation, The Associated Press reported.

was expected in federal court after his aide's arrest on probably cause for possession and distribution of child pornography.

The Republican senator said that he was "stunned, surprised and disappointed" by the allegations against Loskarn, a man seen as "a rising star who had spent the past decade working his way up to increasing important posts in the U.S. House and then the Senate," The AP reported.

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

Peter Carr, the U.S. Department of Justice spokesman, said that Loskarn war arrested based on probably cause of possession and distribution of child pornography charges and remained in custody pending a court hearing on Thursday.

"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 fulfill his duties as chief of staff," The AP reported.

U.S. Rep. Marsha Blackburn, whom Loskarn also worked for, said she was "shocked, saddened and stunned," about the allegations and investigation.

"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 cooperating with the investigation," The AP reported.