(Reuters) - The flag that flew over the U.S. Capitol on Sept. 11, 2001, was destroyed in a fire at a 9/11 memorial complex in Pennsylvania, authorities said Saturday.

In addition to the American flag, also lost in the fire were personal items of the passengers and crew members who were on board United Airlines Flight 93 when it went down in a field as one of four hijackings by al Qaeda militants on Sept. 11, 2001, the National Park Service said in a statement Saturday.

About 100 'tribute' items left by visitors to honor the passengers and crew killed in the crash were also destroyed by the fire, the park service said.

Some of the objects were at the complex to be photographed, measured and prepared for display for inclusion in a new visitor center, which is under construction and slated to open in late 2015.

In all, the Flight 93 National Memorial's headquarters complex was "a complete loss," according to the park service. The complex was roughly 2 miles from the field where the flight crashed. The three buildings gutted by the blaze held at least 10 percent of the memorial's collection of artifacts, many in boxes designed to be fire proof.

Park Service spokesman Mike Litterst said the service saved the vast majority of the 820 oral histories conducted since 2005 that provide first-person insight into the crash. Additionally, 480 DVDs containing tens of thousands of images survived the fire, he said.

The cause of the fire is under investigation, the service said.

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 the 40 passengers and crew and four hijackers on board.

Flight 93 was one of four hijacked airliners on Sept, 11, 2001. One crashed into the Pentagon outside Washington and two crashed into the World Trade Center in New York, killing nearly 3,000 people.

U.S. authorities have said the actions of the passengers and crew aboard Flight 93 thwarted a planned attack on the U.S. Capitol.

(Reporting by Daniel Kelley in Philadelphia and Carey Gillam in Kansas City, Mo.; Editing by James Dalgleish)