[Read our latest coverage of the pipe bomb found at George Soros’s house.]

The explosive device found in a mailbox at the home of George Soros, the billionaire philanthropist, on Monday afternoon was relatively small, a senior law enforcement official said on Tuesday.

The device was “proactively denotated” by bomb squad technicians from the Westchester County Police Department.

The bomber’s motive remained unclear. Mr. Soros is a favorite target of right-wing groups. He was not home at the time.

The investigation has been taken over by the New York offices of the F.B.I. and the federal Bureau of Alcohol Tobacco and Firearms, said the senior law enforcement official, who spoke on the condition of anonymity because the investigation is open.