Police say they are still not certain what caused a gravestone to topple over, seemingly on its own, and hit a young boy in the head.

MetroSource is reporting that a four-year old was visiting the Albany Rural Cemetery ("ARC") in Menands, New York on Saturday evening, September 6, 2014.  The boy was pinned under the headstone but his family was able to pull him out as emergency crews arrived.

The boy, whose identity has not been released, was brought to Albany Medical Center with what police are describing as a "serious head injury."

Police are still investigating exactly what made the tombstone fall.

Albany Rural Cemetery was founded in 1841 and is a National Historic Landmark located near Albany.   More than 135,000 people are buried there, including the 21st President of the United States, Chester Arthur.

The cemetery is also the final resting place for about 1,030 Civil War soldiers and sailors.