Poland's police say they are investigating the theft of a metal plaque honoring Hitler's opponents which was taken from the site of Nazi leader's World War II bunker, now on Polish territory.

Police spokeswoman Anna Fic said Friday that an investigation is underway.

A tourist guide at the Wolf's Lair, a system of some 200 Nazi bunkers and military barracks, noticed Sunday that the plaque had been unscrewed from its stone support. Honoring resistance against Hitler, the plaque was installed in 2004, on the anniversary of the failed assassination attempt on Hitler by Col. Claus Stauffenberg in 1944.

The Wolf's Lair was Hitler's chief quarters from 1941-44, when the area was in Germany. After the war it became part of Poland and is now a major tourist attraction.