SHANKSVILLE, Pa. (AP) — The Flight 93 National Memorial at Shanksville, Pa., where a plane crashed in the 9/11 terrorist attacks is seeking volunteers for a tree planting day April 27.

The Daily American of Somerset (http://bit.ly/X34e6d ) reports that volunteers, ages 14 and older, must register at the National Park Service website. No walk-ups will be accepted.

Authorities expect to plant 15,000 seedlings on 23 acres.

Volunteers will be supervised by a professional forester, and are advised to prepare for cold, wet and muddy conditions.

The memorial is about 80 miles southeast of Pittsburgh. It marks where a hijacked United Airlines jet crashed on Sept. 11, 2001. Investigators believe the hijackers planned to crash the plane in Washington, D.C., but were thwarted by passengers and crew members. Everyone aboard died.

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Online: http://www.nps.gov/flni