Hairline cracks have been discovered in several of the 140 inscribed black granite panels that make up the Vietnam Veterans Memorial.

National Park Service officials estimated that there are about eight of the horizontal cracks, ranging in length from a minimum of one inch to a maximum of six inches.

Although the exact cause of the cracks is not known, they are barely visible to the naked eye, have not expanded and pose no threat to the structure, according to a partner in the monument's architectural firm.

There is "no reason to be alarmed in any way, shape or form," said Kent Cooper of the Cooper Lecky Partnership, the monument's architects of record.

The monument's designer, Yale University student Maya Ying Lin, who Cooper said had been temporarily employed in his office, could not be reached.

The $8.8 million monument was built by the Vietnam Veterans Memorial Fund Inc., and last Veterans Day, on its second anniversary, was turned over to the federal government. It bears the names of more than 58,000 Vietnam War dead.

The cracks, which begin at the edges of the granite panels, at about waist-high level, and in some cases extend among the names, were described as almost imperceptible.

"You could walk by the Memorial for eight years and look at every name and not see them," said architect Cooper.

This year about 2 million persons are expected to visit the monument, which has quickly become one of the most popular in the city.

Few of the visitors see the cracks and none of them ask about it according to park technician Timothy Trainer, who is stationed there.

The cracks, he said, are "barely visible."

A Park Service spokeswoman said about half a dozen of the cracks are located on panels on the west, or left-hand wall of the V-shaped memorial. The other two or so are to the right of the apex.

Although the cracks in the memorial thus far appear of little consequence, architect Cooper said, "because it's an important structure we have had very good people looking into it."

Photographs have been taken to record the size and location of the cracks, for reference in determining whether changes occur.

The search for a possible cause has ruled out any sinking of the foundation, external stresses or "anybody hitting the memorial," the architect said.

"We've been through it all," Cooper said.

Still under consideration, he added, is the possibility of stress existing inside the granite, which was quarried in Bangalore, India.

Cracks, he added, are "very common" in such relatively brittle building stones as marble and granite.

Authorities said it was their understanding that one of the volunteers who works at the memorial first brought the cracks to their attention.

Cooper said he could not tell how long before that the cracks might have existed.