Complex sentence: In August 2004, after learning that an oil painting that had been stolen during the Holocaust from its rightful owner had by donation found its way into the museum's collection in 1993, the museum returned the art to the heirs of its original owner.
Split into simpler sentences: In August 2004, the museum learned that an oil painting that had been stolen during the Holocaust had found its way into the museum's collection by donation in 1993. The museum returned the art to the heirs of its original owner.