Hundreds of volunteers are again set to plant thousands of trees at the Flight 93 National Memorial.

More than 15,000 seedlings will be planted Friday and Saturday at the western Pennsylvania site.

The new seedlings will cover 23 acres of hillside near where United Airlines Flight 93 crashed in an abandoned strip mine in Shanksville after being hijacked by four terrorists during the Sept. 11, 2001 attacks. Forty passengers and crew died as they crashed the plane while fighting for control with the hijackers.

More than 2,000 volunteers have planted nearly 75,000 seedlings since the projects began in 2012. Organizers hope to reforest the entire site with more than 150,000 trees.

The memorial is located 65 miles southeast of Pittsburgh.