The National Park Service will provide a tour of the visitor center being built at the Flight 93 National Memorial on the eve of the 13th anniversary of the Sept. 11 terrorist attacks.

The memorial marks the spot where United Airlines Flight 93 crashed in a field about 75 miles southeast of Pittsburgh after passengers fought back against hijackers.

A permanent memorial was dedicated on the 10th anniversary of the attacks. Last year, the Park Service awarded a $20 million contract to URS Group Inc. of Morrisville, North Carolina, to build the visitor center and do other site improvements.

The visitor's center is expected to open late next year. Officials hope it will increase the number of people who visit the memorial annually from about 300,000 to 500,000.