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.
Federal and state law enforcement officials responded to the scene in Katonah, N.Y., a hamlet in the upscale town of Bedford in northern Westchester County, after the Bedford Police Department received a call about a suspicious package at about 3:45 p.m.
“An employee of the residence opened the package, revealing what appeared to be an explosive device,” the police said in a statement. “The employee placed the package in a wooded area and called the Bedford police.”
Mr. Soros, who was born in Hungary, made his fortune running a hedge fund and is now a full-time philanthropist and political activist. He donates frequently to Democratic candidates and progressive causes and has given at least $18 billion to his Open Society Foundations to promote democracy and human rights around the world.
His activism has made him a villain to conservative groups and the target of anti-Semitic smears. Roseanne Barr called him a Nazi in an infamous tweetstorm, and just this month, Representative Matt Gaetz, Republican of Florida, falsely speculated that Mr. Soros had funded a caravan of migrants moving north toward Mexico. There is no evidence that Mr. Soros paid thousands of migrants to storm the border. Nor is there evidence that Democrats support the effort, as President Trump has said.
Mr. Soros became a major political donor in the United States during George W. Bush’s presidency. He spent millions backing John Kerry’s unsuccessful bid to deny Mr. Bush a second term, was an early backer of Barack Obama’s 2008 presidential campaign and contributed more than $25 million to Hillary Clinton and other Democratic candidates and causes during the 2016 election cycle.
Though Mr. Soros’s name has become a trigger for a subset of Republicans and conservatives, he has said that his main goal as a political activist was to see a return to bipartisanship.
He has said it was the extremism of the Republican Party that had prompted him to become a major Democratic donor, but also said he is opposed to the extreme left.
“I don’t particularly want to be a Democrat,” he said.
Bedford is about 50 miles north of Manhattan and has been home to many well-to-do city transplants, including Martha Stewart, Glenn Close and Ralph Lauren. Katonah, where Mr. Soros has a home, is known for its cultural offerings, including the Caramoor Center for Music and the Arts and the Katonah Museum of Art.
The town is known for its insularity and lack of gossip, which allows for well-known residents to fly under the radar. “They look to Bedford as to being protective of their privacy,” John Stockbridge, Bedford’s historian, said in a phone interview.
Mr. Stockbridge said that the town has largely avoided drama or violent scares. “I’ve been here for 35 years and I don’t remember any incident like what you’re talking about,” he said.