SHANKSVILLE, Pa. (AP) — A fire at the Flight 93 National Memorial on Friday badly burned three administrative buildings, a National Park Service spokesman said.

All employees evacuated safely, and no injuries were reported, spokesman Mike Litterst said. He did not immediately know the cause of the fire but said in a statement that initial reports indicated "extensive damage" to the complex.

The affected buildings serve as the park's headquarters and include the superintendent's office, Litterst said. The flames didn't affect the memorial or construction of the visitors center, which are about 2 miles away on the large property.

The memorial in Shanksville marks the spot where United Airlines Flight 93 crashed during the Sept. 11, 2001, terrorist attacks. The plane, which was traveling from Newark, New Jersey, to San Francisco, went down in a reclaimed strip mine after passengers fought back against its hijackers. All 33 passengers and seven crew members were killed along with the hijackers.

A memorial plaza was completed in time for the 10th anniversary of the attacks in 2011. It features a white stone wall with separate panels for each victim, with one name engraved on each.

The wall traces the path of the doomed flight. The wall and 40 groves of 40 trees are ways to focus attention on the crash site and the victims' memories.

Officials have said they hope construction of the visitors center will be finished by June. That would give park officials three months to install exhibits in time to open for the 14th anniversary of the crash.