Police and the FBI are investigating the disappearance of two pieces of artwork worth more than half a million dollars from the Boston Public Library, and a top employee has been placed on leave.

It's still not clear whether the works by Rembrandt and Albrecht Durer were stolen or were simply misplaced.

Police Commissioner William Evans says investigators are looking into whether the disappearance of the pieces was "an inside job" and are looking at several library employees. He did not disclose names.

A library spokeswoman says the keeper of special collections is on leave.

The missing pieces are a 1634 Rembrandt etching and the 1504 Durer engraving.

The 8-by-11-inch Durer, "Adam and Eve," is valued at about $600,000. The 5-by-6-inch Rembrandt self-portrait is valued between $20,000 and $30,000.