[Read our coverage of the devices sent to Hillary Clinton, Barack Obama and CNN’s office in New York. They were similar to the one sent to George Soros.]

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 vitriol 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.