NORTH RIVERSIDE, Ill. (AP) _ A 4-year-old boy who was visiting the grave of his grandmother with his parents on Mother's Day was crushed to death when a 500-pound headstone fell on him, police said Monday.

Brandon Schwartz died despite a heroic rescue effort by his father, Marvin, who lifted the heavy granite stone off his son, said Sgt. Robert Slak. When police arrived on the scene, it took four officers to move the marker.

''You can imagine what kind of strength it would take for one person to do that,'' said Slak. ''You can also imagine the shock at seeing your son pinned beneath it.''

The toppled headstone, which was not that of Brandon's grandmother, was mounted on a concrete base, the back of which apparently had settled into the ground and left the marker listing at an angle.

Authorities were waiting for a final report Monday from the medical examiner's office to determine the exact cause of Brandon's death.

Sgt. John O'Rourke said Sunday that Jewish Waldheim Cemetery ''is one of the older cemeteries with high tombstones. No one knew how it fell or why it fell.''

According to O'Rourke, the boy was walking behind his parents when Schwartz said he heard a thud and turned and saw his son under the gravestone.

The child was pronounced dead at Loyola University Medical Center in Maywood, said spokeswoman Pat Hartnett.

''It's just unbelievable,'' said Sylvia Benish, a neighbor of the Schwartzes. ''He was so handsome and so smart.''