Authorities believe they have found a package addressed to former Vice President Joe Biden that was considered suspicious due to its similarities to other packages sent this week to prominent Democrats and CNN, law enforcement officials said Thursday.
The package was misaddressed and returned to sender, two law enforcement sources previously told CNN.
Also on Thursday, a suspicious package addressed to actor-director Robert De Niro was reported at the Manhattan building where his production company is based, and its marking and contents appear similar to pipe bomb packages recently mailed nationwide to top Democrats, two law enforcement sources said.
Law enforcement authorities are treating the series of bombs as a domestic terror matter and are advising the public to remain vigilant.
Devices sent to former President Barack Obama, Hillary Clinton, former Attorney General Eric Holder, California Democratic Rep. Maxine Waters and Democratic donor George Soros showed the presence of a sulfur substance, which could have exploded, a law enforcement official said.
The devices are believed to be pipe bombs, inherently unstable, and at risk of being set off just by handling.
The FBI's counterterrorism division is leading the investigation Thursday into the packages and the agency says it's possible additional packages were mailed to other locations.
The motive is unknown, but the recipients are all prominent targets of right-wing criticism and, in many cases, of President Donald Trump himself. The package found at CNN's New York bureau in the Time Warner Center was addressed to former CIA Director John Brennan, city and local law enforcement officials said.
At the White House, Trump vowed Wednesday that "we will spare no resources or expense" to pursue the perpetrator.
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