"After what happened here . . . it's tacky and unpatriotic," said JoAnn Marquis, visiting the site with her husband from Salem, Mass.

The panels outline the twin towers' history, including their construction, the 1993 bombing and their ultimate destruction by terrorists on Sept. 11, 2001.

The diagrams and photos are now covered with writing.

Some of the messages are words of hope, remembrance or prayer; others are indecipherable or vaguely crude. Many visitors to the site said they considered even positive messages inappropriate.

"Graffiti is graffiti," said Rachael Jackson of Las Vegas. "It angers me."

Steve Coleman, a spokesman for the Port Authority, which handles maintenance of the viewing wall, said agency officials are aware of the graffiti and appalled by it.

"We're very disturbed and disappointed that anyone would think of defacing anything at the World Trade Center site," Coleman said. The agency is having replacement panels made, he said. They will be up by Labor Day.