A bomb was found in a post box at the suburban New York compound of billionaire philanthropist George Soros, who has been the target of conspiracy theorists.
Federal investigators were reviewing surveillance video to determine whether the package had been sent through the post or delivered another way.
A security officer at the compound about 50 miles north of Manhattan became suspicious of the package on Monday afternoon and placed it in a nearby wooded area before alerting the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives.
“It was not a hoax device,” said an official.
Another federal official said the device resembled a pipe bomb and was inside a package placed in a post box outside the gates of the compound. It was opened in a secure location just inside the gates, nowhere near Mr Soros’ quarters, the official said.
The Bedford Police Department said the FBI’s terrorism task force was investigating.
The FBI’s New York field office said on Twitter that there was “no threat to public safety”.
Mr Soros, who made his fortune in hedge funds, frequently donates to liberal causes.
Mr Soros, who is Jewish, has also been the target of anti-Semitic smears. Some have falsely accused him of being a Nazi collaborator during the Second World War, when he was a child in Hungary.
Activists frequently post on social media the addresses of his homes, sometimes along with threats.