WASHINGTON (AP) - A historian and former director of the National Portrait Gallery is returning to the Smithsonian Institution to serve as acting director of the National Museum of American History.

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Marc Pachter retired from the Smithsonian in 2007. On Wednesday, the museum complex announced he will return Aug. 15 to temporarily replace Brent Glass, who is leaving the American history museum.

Glass announced in June that he will step down as director after nine years.

Richard Kurin, the Smithsonian's undersecretary for history, art and culture, says Pachter will lead one of the busiest museums on the National Mall until a permanent director is found. Pachter previously served as the museum's interim director in 2001 and 2002 and oversaw a commemorative exhibit about the Sept. 11 attacks.