Investigators said they hope to solve a cold-case homicide, but declined to give further details. Police sources tell NBC New York the murder dates to about 10 years ago.
The NYPD crime scene unit, State Police and investigators with the medical examiner's office were on scene for several hours, digging for remains.
Search dogs also combed the woods along the Hudson.
In addition to the Riverdale Country Day School, the search area is also near Wave Hill and a wealthy neighborhood of million-dollar homes.
The search concluded midday Monday; investigators planned to return Tuesday.