>> Read more trending stories

Among items destroyed in the fire were a flag that had flown over the U.S. Capitol on Sept. 11, 2001, and a handful of personal items of the passengers and crew of Flight 93, some of which had been recovered from the crash site and others that had been provided by family members. Copies of DVDs of the annual commemoration ceremonies and meetings of the Flight 93 National Memorial Federal Advisory Commission, as well as 100 tribute items that had been left by visitors since 2001, were also lost.

Litterst said the majority of the 60,000 tribute items that had been left at the memorial were unaffected, having been stored offsite. In addition, most, if not all, of the 820 oral histories of Flight 93 and the memorial's archival photo collection were spared by the flames.

The investigation into the fire is being conducted by the National Park Service and Pennsylvania State Police.