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An unnamed senior Democratic staffer confirmed as much in a July 31, 2009, e-mail to a Republican staff member, according to Mr. Issa. The staffer wrote, “That was my impression too, that she was telling us she had nothing to do with [IG Weiderhold’s removal] and [his removal] was news to her.”

Asked to respond to Mr. Issa’s letter, Amtrak officials said in a statement Friday that the rail agency has cooperated with “numerous entities” that have reviewed Mr. Weiderhold’s departure.

Amtrak will cooperate fully with any other inquiry into this matter,” Amtrak said in the statement.

Mr. Issa’s letter also said Amtrak was being investigated by the U.S. Postal Service’s Office of Inspector General for possible misuse of Amtrak’s e-mail system.

Mr. Carper was named Amtrak chairman in January 2009, after about a year on its board of directors. He was mayor of Macomb, Ill., from 1991 to 2003. Ms. Acheson, a big Democratic donor, was an assistant attorney general in the Justice Department during the Clinton administration and has worked in private practice.