SHANKSVILLE, Pa. -- The National Park Service yesterday covered a cash donation box at the Flight 93 crash site that caused consternation among families of the victims of the Sept. 11, 2001, crash.

"The bottom line is we feel the National Park Service can't effectively carry out our mission without exercising the exclusive use and control of the site as provided for in our agreement with the property owners," Flight 93 Memorial Superintendent Joanne Hanley said.

A black plastic trash bag was placed over the cash box the property owner, Mike Svonavec, installed near a temporary memorial to cover security costs. Security has been handled by a private firm at a monthly cost of about $10,000 since federal funding ran out in February, and Mr. Svonavec said the site has also incurred other costs.

"I don't understand what their basis is," Mr. Svonavec said of the decision to cover the box. "I've cooperated with everyone. Obviously, they are exercising the power the National Park Service feels it has."

Victims' families did not like the box, calling it an insult to the 40 passengers and crew members who died when the hijacked Newark, N.J.,-to-San Francisco flight crashed in the field near Shanksville.