The package was found Saturday night in the tony Bel Air neighborhood after it was dropped off at a neighbor's house.
The Los Angeles Police Department's bomb squad was called to the home and officers opened the box, finding a pile of horse manure inside. Police said the package had been gift-wrapped and was marked as being from "the American people."
Police said the Secret Service was taking over the investigation. A Secret Service spokesman said the agency was aware of the incident but declined to comment further.
A spokesman for the Treasury Department did not immediately respond to a request for comment Sunday.