Over a 72-hour period, devices were also discovered addressed to prominent liberal critics of President Trump, including former president Barack Obama, former secretary of state Hillary Clinton, and Rep. Debbie Wasserman Schultz (D-Fla.). A Wasserman Schultz district office in South Florida was listed as the return address on the packages, in what current and former investigators said was a feint suggesting particular antipathy on the part of the bomber toward her.

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One lead being pursued by investigators is that some of the devices may have been mailed from South Florida, but officials were cautious Thursday and urged the public to call in with tips from anywhere.

The packages had many of the hallmarks of suspicious mail, including large block lettering and excessive postage aimed at making it harder to track, said Matthew Doherty, who formerly led the U.S. Secret Service’s National Threat Assessment Center. The fact that none detonated provides investigators with considerable evidence, he said.

“There’s a rich treasure trove of forensic information since they were found intact,” Doherty said. That means FBI investigators can “look for patterns such as the device, the technical expertise, the method of mailing, a whole host of great, rich forensic evidence that can be gathered.”

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The bomb packages have been found in five states — New York, Delaware, Maryland, Florida, and California — and the fourth day of the investigation began in the early morning hours when a retired NYPD detective who now works for De Niro saw an image of one of the suspect devices on the news.

“It struck him that that looked very much like a package he had seen on Tuesday for mail he was to screen for Robert De Niro Productions,” said John Miller, the NYPD’s deputy commissioner of intelligence and counterterrorism. De Niro, an Oscar-winning actor, has publicly clashed with Trump, including using an expletive to denounce the president at this year’s Tony Awards on Broadway.

The bomb squad was called to De Niro’s offices in Lower Manhattan, and the device was found and removed by 6:30 a.m., officials said. Hours later, the FBI announced it had found two similar packages in Delaware. Both were addressed to Biden, but neither had made it to his home, officials said.

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Authorities appealed for tips at a Thursday afternoon news conference in New York.

“We are investigating all of this with great precision, and I can say with certainty that we will identify and arrest a person or people responsible for these acts,” NYPD Commissioner James P. O’Neill said.

Officials declined to say whether the devices were intended to detonate or were meant to scare people, but they repeatedly urged the public to view them as if they could pose a threat.

“We are treating them as live devices,” said O’Neill, urging people not to touch packages they deem suspicious. “This is something that should be taken seriously.”

William F. Sweeney Jr., assistant director in charge of the FBI New York field office, asked people to remain vigilant, warning that more devices “have been or could be mailed.” He said the powder contained in envelopes holding the devices found in New York did not pose any biological threat and said the substance was still being analyzed.

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The U.S. Postal Service has inspectors who are trained to recognize suspicious mail and operate X-ray machines as part of its screening process, officials said. Philip Bartlett, inspector in charge of the U.S. Postal Inspection Service’s New York division, said Thursday afternoon that the Postal Service had found nothing since early that morning, leaving the tally at 10 packages.

The devices have prompted heightened security nationwide. Police have increased patrols of high-profile people, areas and organizations, and authorities have warned some prominent figures — including former president Jimmy Carter — to be on alert.

While the first explosive device was found Monday — at the New York home of George Soros, a billionaire activist known to fund pro-democracy and liberal political groups — it was not until early Wednesday that it became apparent investigators were hunting a serial bomber. The Secret Service said it had intercepted two bombs — one addressed to Clinton at the New York home she shares with former president Bill Clinton and another addressed to Obama in Washington that were found at mail screening facilities. Neither bomb included a note, law enforcement officials said.

Officials described the devices as PVC pipes stuffed with what appeared to be fireworks powder and glass. Electrical wires leading out of the pipe attached to an electric timer taped to the side, according to law enforcement officials speaking on condition of anonymity to discuss the investigation. Most of the devices appeared to have been sent through the mail system.

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The FBI said the packages found so far had shared characteristics, including manila envelopes with bubble-wrapped interiors. They all also had a half-dozen Forever stamps, computer-printed address labels and return addresses bearing the misspelled name of Wasserman Schultz, who chaired the Democratic National Committee during part of the 2016 presidential campaign. Officials do not think she had anything to do with the packages and consider her a likely target.

Other packages were soon discovered, including two sent to Rep. Maxine Waters (D-Calif.); one sent to CNN’s New York offices and addressed to John Brennan, the former CIA director; and another addressed to Eric H. Holder Jr., Obama’s first attorney general. That package had an incorrect address, so it was redirected to Wasserman Schultz’s office in Sunrise, Fla., and recovered there.

At least one of the packages sent to Biden also had an incorrect address and was being redirected to Wasserman Schultz when investigators intercepted it in Delaware, officials said.

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All of the bomber’s targets have clashed sharply with Trump at different times, and the spate of dangerous packages intensified the already full-throated political fights two weeks before congressional elections.

On Wednesday, Trump spoke out against violence and violent rhetoric. On Thursday, he tweeted: “A very big part of the Anger we see today in our society is caused by the purposely false and inaccurate reporting of the Mainstream Media that I refer to as Fake News.”

John Wagner, Katie Zezima and Matt Zapotosky contributed to this report.