Fire broke out on Friday at the Flight 93 National Memorial's headquarters complex in Pennsylvania, the National Park Service said, causing no reported injuries but extensive damage near the site of the Sept. 11, 2001, crash of a hijacked airliner.
There was no damage to the memorial site in a field where a United Airlines flight went down in one of four hijackings staged by al Qaeda militants on that day. The headquarters complex is two miles (3.2 km) away from the crash site.
A National Park Service spokesman earlier told KDKA, the CBS affiliate in Pittsburgh, that he did not know the cause of the fire at the headquarters complex.
The 2,200-acre (890-hectare) memorial park near Shanksville, about 80 miles (130 km) southeast of Pittsburgh, features a wall of names that partially surrounds a field where the flight went down, killing all 44 people on board, including four hijackers.
A visitors center currently under construction is slated to open in late 2015. The Park Service said the fire did not affect the center.
On the same day of the Flight 93 attack, hijacked airlines crashed into the Pentagon outside Washington, D.C., and the World Trade Center in New York. Some 3,000 people died.
(Reporting by Daniel Kelley in Philadelphia; writing by Frank McGurty; editing by Scott Malone and Eric Beech)