ALBANY — Amid a day of fear caused by a series of explosive devices sent to Democratic politicians as well as the Manhattan offices of CNN, Gov. Andrew M. Cuomo on Wednesday said a suspicious package had been sent to his office in New York City.

The truth followed within an hour: The package contained nothing more than a thumb drive containing material on the alt-right group known as the Proud Boys, according to Cuomo's spokesman. It was not immediately clear who had sent it.

The Proud Boys recently inflamed controversy after a brawl that followed a speaking appearance by its founder, Gavin McInnes, at the headquarters of the Metropolitan Republican Club. The melee has so far resulted in the arrests of a half-dozen members of the white nationalist group.

According to a photo of the package posted by Zack Fink of NY1, it was addressed to Cuomo's Manhattan office with the identical return address. It also said "Proud Boys Info" in the lower left.

Cuomo mentioned the package had been taken away by authorities as he appeared alongside New York City Mayor Bill de Blasio at a press conference with law enforcement officials responding to the discovery of an apparent explosive device at the studios of CNN.

Just days after a pipe bomb was dropped off at the home of financier George Soros, the U.S. Secret Service on Wednesday reported intercepting similar packages sent to former President Barack Obama and former Secretary of State and New York U.S. Sen. Hillary Clinton.

Soros, Obama and Clinton are among the biggest targets of right-wing conspiracy theorists, while CNN has been a favorite target of President Donald J. Trump and his supporters. Cuomo's brother, Chris Cuomo, is a prime-time CNN anchor.

Gov. Cuomo said what he described as a "device" was being "handled as we speak." The governor never described the package as an explosive.

Speaking to CNN just before his press conference, Cuomo said political divisiveness was breeding terroristic behavior.

"Let's have our political differences with civility. ... We are at a boiling point," he said.

The governor told his brother during the CNN interview that all of the suspicious packages were "manila envelopes with an explosive device inside." He said the Westchester Police Department exploded the device sent to Soros "in a controlled circumstance" and the other packages have been seized by the FBI.

"There is no doubt this is a terrorist attack writ large," the governor said.

Cuomo has deployed 100 additional National Guard soldiers to the New York City metro area and asked the State Police to also double their security at major locations in New York. A news release from his office listed downstate airports, mass transit systems, bridges and tunnels as the site of the larger law enforcement presence.