SOMERSET, Pa. Designers are considering planting a variety of trees at the Flight 93 memorial, rather than just one species, to prevent disease and insect infestation.

"We have 40 groves of 40 trees. If we plant just one species, it may be more susceptible to pests and diseases," said Joanne Hanley, Flight 93 National Memorial superintendent.

Central to the design of the memorial is an arc of maple trees surrounding a walkway around a natural bowl leading to the crash site, about 70 miles southeast of Pittsburgh.

The trees are part of a $22.5 million initial phase of the memorial expected to be dedicated on the 10th anniversary of the terrorist attacks.

United Airlines Flight 93 was en route from Newark, N.J., to San Francisco when it was hijacked on Sept. 11, 2001. An official investigation said the hijackers crashed the plane in the Somerset County field when passengers and crew tried to wrest control of the cockpit, killing all 40 passengers and crew, as well as the four hijackers.