Several locations in the Washington, D.C., and NY areas and Florida received suspicious packages this week.
X-ray images show that the package addressed to De Niro contains a device similar to the pipe bombs found in other packages this week that were addressed to prominent Democrats, the two sources said.
The list of bombing targets spread from New York, Delaware and Washington, D.C., to Florida and California. The package appears similar to pipe bomb packages recently mailed to top Democrats nationwide.
None of the eight packages detonated and nobody was hurt.
President Trump responded quickly, condemning the attempted attacks. The five packages discovered Wednesday all had a similar return address: That of Democratic Congresswoman Debbie Wasserman Schultz, but with her name misspelled.
Malcolm Brady, a former Bureau of Alcohol, Tobacco, Firearms and Explosives official and bomb investigator, told the Post that suspects of such crimes are often "loner types" and suggested the wrong address on Holder's package may have been intentional.
"The protectees did not receive the packages, nor were they at risk of receiving them", Secret Service said in the statement.
Hillary Clinton: The package was addressed to her residence in Chappaqua, New York, and was intercepted Tuesday night by the Secret Service during routine mail inspection.
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All the people who were targeted are frequently maligned by right-wing critics, including former U.S. president Barack Obama, former secretary of state and presidential candidate Hillary Clinton and Obama's attorney general Eric Holder. It matches the description of those sent to the other officials. On Wednesday, we learned that explosive devices were mailed to the Obamas, the Clintons, and CNN, a day after George Soros also received a crude pipe bomb in the mail. The former Central Intelligence Agency director frequently appears on CNN.
A suspicious package addressed to former Vice President Joe Biden was found at a mail facility in New Castle County in DE, a federal law enforcement official told Reuters.
The actor owns a restaurant in Manhattan's upmarket Tribeca neighbourhood, where the package was found, and has been critical of Trump in the past.
The package intended for Holder had the wrong address and was forwarded to Wasserman Schultz.
That person, who has a background in law enforcement, recalled seeing a similar package at the film center mailroom earlier and reported it to police.
The first package turned up on Monday and was addressed to Soros, the billionaire financier and advocate of liberal, open-border values who is a frequent target of right-wing conspiracy theories.
The news organisation's NY staff were evacuated.
"A package containing a pipe bomb was sent to Brennan but addressed to CNN's NY office".
The FBI confirmed there was law enforcement activity at the USA mail facility in DE and aerial footage shot by NBC showed a heavy law enforcement response. The packages are believed to be "pipe bombs", volatile by their very nature.