The Hill 101: There is no "Capitol Jail"
When discussions in Congress turn to holding someone in contempt, there is invariably talk about a supposed "jail in the basement of the Capitol." This story was given new life last week by Nancy Pelosi suggesting that Karl Rove could have been jailed in the Capitol in 2007. (The New York Times thought so too.)
Is there really a jail in the Capitol basement? No.
Lucky for us, POPVOX developer and former Congressional staffer, Annalee Flower Horne, is a master of Capitol-related trivia. She has given hunderds of Capitol tours and studied the building since she was a kid.
There is no Capitol jail.
There is a tomb chamber under the crypt (beneath the star), which was meant to house Washington's body. Washington made arrangements to be buried at Mount Vernon. The tomb chamber has bars, leading many to assume it must be a jail cell. It's never been used as one, though.
I believe the last person to receive the dubious distinction of an arrest for contempt of Congress was a member of the Hoover administration--but they didn't hold him in the Capitol. He was held at the Willard Hotel. These days, if Congress needed to jail someone, they'd probably use the Capitol Police's offsite holding cell.
There was, however, a building called the "Old Capitol Prison," on the grounds of what's now the Supreme Court building. It was the temporary Capitol for a few years after the British set the real one on fire in 1814. After that, it was used as a private school, then a boarding house (where Vice President Calhoun died in 1850). It became a prison during the Civil War. Later, it served as the headquarters for the National Women's Party before the Federal Government bought it back and razed it to make room for the Supreme Court.
Check out this interview from the Senate Historical Office oral history project: Towards the bottom of Page 20, they talk about contempt and rooms in the Capitol used at various times to hold prisoners.
CNN covered the question in 2007: