Federal authorities believe that an explosive device found Monday in a mailbox at the home of George Soros, the billionaire philanthropist who has been a focus of right-wing critics and conspiracy theorists, was left there by someone and was not delivered by the Postal Service, several law enforcement officials said Tuesday.

Mr. Soros’s home is in a suburb of New York City. The device was constructed from a length of pipe about six inches long filled with explosive powder, and it was “proactively detonated” by bomb squad technicians, according to one of the officials, all of whom were briefed on the investigation.

The motive of the would-be bomber or bombers remained unclear, one of the officials said, adding that there had not yet been any claim of responsibility.

Mr. Soros, who made his fortune in finance and is now a full-time philanthropist and political activist, is often a subject of the ire of right-wing groups. In recent days, some have falsely speculated that he funded a caravan of migrants moving north in Mexico.

Mr. Soros was not home when the device was discovered by a caretaker, another one of the officials said. It was rigged with a detonator, and it could have maimed or possibly killed someone had it exploded near them.

A senior law enforcement official described the pipe bomb as “smaller than what we typically see.”

The investigation is being conducted by the New York offices of the F.B.I. and the federal Bureau of Alcohol Tobacco and Firearms, according to several of the officials, all of whom spoke on the condition of anonymity because the investigation was ongoing.

I’m thinking that these members of the “Party of God” (in Arabic, “Hezbollah”) need to be addressed sooner, rather than later.