An oily substance found splashed on the Vietnam Veterans Memorial this month was the result of vandalism, U.S. Park Police determined.

Sgt. Robert Lachance, a Park Police spokesman, said Monday that a detective made the conclusion, but he declined to provide further details because of the investigation is ongoing.

The Vietnam Veterans Memorial Fund, which built the structure, has offered a $5,000 reward for information leading to an arrest and conviction in the case.

The oily substance on the black granite wall — which bears the names of more than 58,000 men and women killed or missing in the Vietnam War — was first reported Sept. 7. National Park Service officials said they did not know what the substance was, and at first said it was unclear whether it was the result of vandalism or some kind of accident.

On Monday, dark blotches remained along a stone curb at the base of the wall for much of its length. At least 14 of its 140 inscribed panels appeared to be stained.

Park Service spokesman Bill Line said maintenance and preservation crews were working to remove the stains and marks but were proceeding cautiously to avoid further damage. He said officials are confident they can remove all the stains, but it could take a week or more.

"It's deplorable that someone would vandalize what's really a national shrine," said Jan C. Scruggs, founder and president of the Memorial Fund. "It's an outrage. It's sad."