U.S. park rangers say a vertical scratch has been discovered in the Vietnam Veterans Memorial that is deeper than scars detected earlier this year.

The latest scratch, four inches long and an eighth of inch wide, does not touch any of the 58,156 engraved names. The highly polished granite wall is so hard that National Park Service officials said the damage only could have been inflicted deliberately.Park Ranger James Lance, who served in Vietnam in a non-combatant role, discovered the damage Sunday morning but said he was not sure when the damage occurred.

Officials said vandalism is a difficult security problem for rangers and U.S. Park Police because people routinely touch the wall and make rubbings of names.

The memorial is the most popular in Washington, and has been since it opened in 1982, attracting 12,000 to 15,000 visitors daily.

"The problem is the nature of the memorial," said Jan Scruggs, president of the Vietnam Veterans Memorial Fund that jointly administers the memorial with the Park Service.

"What makes it successful is the feeling people get from touching it, from touching the names. We would never consider covering it with glass to protect it."