Updated at 9:58 a.m. ET
Federal investigators are searching on Thursday for whoever sent a wave of suspicious packages addressed to political nemeses of President Trump.
About nine of the envelopes, some of which have contained homemade pipe bomb-like potential explosives, have been intercepted or recovered by authorities.
The latest included a suspicious package addressed to former Vice President Joe Biden, which was found in New Castle County, Delaware, a law enforcement official said. The package was similar to the other suspicious packages discovered in recent days.
And New York City police said a suspicious package was discovered in the Tribeca area of Manhattan in the early hours of Thursday morning. Officials confirmed the discovery to NPR, saying the matter is under investigation. That package contained a device similar to one recovered from the offices of CNN on Wednesday.
The package was found "at the business residence of Robert De Niro," New York Gov. Andrew Cuomo told NPR's David Greene. The Tribeca Grill, one of De Niro's restaurants, is at the corner of Greenwich and Franklin Streets.
The New York Police Department says the package was reported around 4 a.m. in the area of Greenwich and Franklin streets in Tribeca. The police bomb squad responded, and it was "removed from the location," the NYPD says.
De Niro has been an outspoken critic of Trump, including in an expletive-laced speech at this year's Tony Awards. Trump shot back at him on Twitter.
Suspicious packages also were sent to former President Barack Obama and former Secretary of State Hillary Clinton.
Others were addressed to former CIA Director John Brennan in care of CNN in New York, former Attorney General Eric Holder and Rep. Maxine Water, D-Calif. The first package was reported on Monday, at the residence of billionaire activist George Soros.
Cuomo said the two suspicious devices found in New York City shouldn't stop people from going about their business as normal.
"Don't let the terrorists win," he said. "There's no reason to be afraid."
Investigators are now working in New York and other cities, within the postal system, and at the federal level, to determine who sent the packages — and why.
Officials responded to a mail processing center in Delaware after one package addressed to Biden was recovered there.
And the Secret Service said suspicious packages were addressed to Clinton in Westchester County, N.Y., and to Obama's residence in Washington, D.C.; those packages contained explosive devices that were similar to one that was found on Monday at the home of George Soros — a frequent contributor to Democratic and progressive causes.
An explosive ordnance disposal team destroyed the package sent to Soros through a controlled detonation.
Trump said the inquiry into the packages will have whatever resources are needed and FBI Director Christopher Wray pledged that the bureau would not rest until the sender or senders are caught.
"This investigation is of the highest priority for the FBI," Wray said. "We have committed the full strength of the FBI's resources and, together with our partners on our Joint Terrorism Task Forces, we will continue to work to identify and arrest whoever is responsible for sending these packages."
But the FBI also has acknowledged that more suspicious packages may still be working their way through the mail and there is no way to know now how many there might ultimately be.
NPR's Hansi Lo Wang contributed reporting to this story.