Harry Hamburg/AP Photo

The chief of staff to Sen. Lamar Alexander, R-Tenn., was arrested on child pornography charges today by the U.S. Postal Inspection Service, a Justice Department spokesman said.

Law enforcement agents searched the home of Ryan Loskarn, 35, this morning and he was taken into custody based on "probable cause for possession and distribution of child pornography," authorities said.

Loskarn will remain in custody until his court appearance, which is expected to be Thursday.

Alexander fired and replaced Loskarn this afternoon.

"The courts will judge Mr. Loskarn's guilt or innocence, but under these circumstances, he cannot continue to fulfill his duties as chief of staff of this office," Alexander said in a statement. "Therefore, as of today, I have removed him from the payroll."

Earlier today, Sen. Alexander placed Loskarn, who was the chief of staff in Alexander's Washington, D.C. office, on administrative leave without pay when he became aware of the investigation, the senator said in a statement.

"I am stunned, surprised and disappointed by what I have learned," Alexander said. "The office is fully cooperating with the investigation."

The U.S. Postal Inspection Service is the lead investigative agency in this case, said Lori McCallister, the U.S. Postal Inspector.

"All documents related to this law enforcement action are currently under seal, therefore no additional information will be provided at this time," McCallister said in an e-mail.

Loskarn has worked on Capitol Hill for years, previously working in the offices of former Sen. Jon Kyl, R-Ariz., and Rep. Marsha Blackburn, R-Tenn.

Alexander has named a new chief of staff, David Cleary, who has served as his legislative director since 2011.

Loskarn did not immediately respond to a request by ABC News for comment.

ABC News' Jack Cloherty contributed to this report.