Officials rushed to respond to reported suspicious packages from San Diego to Boston, only to determine they were not dangerous.

Investigators in a Delaware mail facility found a package addressed to Biden similar to the others that contained pipe bombs, according to a law enforcement official, pushing the search for the bomber into its fourth day.

The package was apparently not delivered to Biden’s home and may have been on its way to being returned to the person listed on the return address of the envelope, the official said. A second package sent to Biden, similar to the others, was also found in Delaware, the FBI said.

The FBI declined to elaborate beyond saying that local and state police, along with US Postal Service investigators and FBI agents, were “responding this morning to a United States Postal facility in Delaware to conduct law enforcement activity”.

A similar package was also found addressed to De Niro, who has publicly clashed with Trump, at his production offices in Manhattan, according to the authorities.

This package was discovered around 5am by security personnel working for De Niro’s film company, Tribeca Productions, police said. A police spokesman said the parcel “was similar to explosive packages that had been publicised”.

An X-ray of the package indicated a similar pipe bomb inside, and investigators suspect it was sent by the same person who sent explosives to Clinton, Obama and others, a law enforcement official said.

Video from the scene showed the NYPD bomb squad removing the package in a heavily armoured containment truck. It was transported to Rodman’s Neck, a peninsula in the Bronx where the NYPD detonates bombs, the department said.

Officials warned that additional devices could turn up. “It’s prudent and diligent to expect more,” New York Governor Andrew Cuomo, a Democrat, said Thursday on MSNBC.

New York Mayor Bill de Blasio, a Democrat, urged people across the city to be on alert, and to refrain from opening suspicious packages. “Alert authorities, call 911,” de Blasio said on CNN. He added that New York had “put an additional NYPD presence outside media companies across the city, outside the offices of certain elected officials”.

Attorney General Jeff Sessions on Thursday morning decried the devices and said the Justice Department was working with other agencies in responding.

“Across the administration, we condemn the actions that led to these activities in the strongest possible terms and are determined to make sure that we get to the bottom of it and justice is done,” Sessions said.

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 that personnel conducting standard mail screenings late Tuesday found a pipe bomb addressed to Clinton at the New York home she shares with former president Bill Clinton. Hours later, a similar package was found addressed to Obama. Neither bomb included a written message, law enforcement officials said.

Law enforcement officials described the devices as pipes stuffed with explosive material and wrapped in electrical wire and tape, but they provided no detail on how they would have been detonated.

At least two of the devices appeared to have been hand-delivered, according to law enforcement officials, while others were found in the mail.

The FBI said the packages found so far had shared characteristics, including manilla envelopes with bubble-wrapped interiors.

They all also had a half-dozen US stamps, computer-printed address labels and return addresses bearing the misspelled name of Representative Debbie 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 believe she was a possible victim. Wasserman Schultz, in a statement. decried “this appalling attack on our democracy”.

Other packages were soon discovered. One was sent to CNN’s New York offices and addressed to John Brennan, the former CIA director. Brennan, a regular cable news commentator and strong critic of Trump, is a contributor to MSNBC and NBC News, not CNN.

That prompted an hours-long evacuation of the CNN offices and others at the Time Warner Centre, sending them flooding onto the streets below. Another was sent to Eric Holder Jnr, Obama’s first attorney general, but an incorrect address was used, so it was “returned” to Wasserman Schultz’s office in Sunrise, Florida, and it was recovered there. A spokesman for Holder declined to comment.

As more devices were found, a disturbing pattern emerged – the bomber or bombers appeared to be targeting prominent Democratic figures and others who have clashed with Trump and been the focus of his incendiary rhetoric. De Niro declared “F*** Trump” during remarks at the Tony Awards over the summer; the president responded by tweeting that he was “a very Low IQ individual.”

Trump has used similar language to describe Representative Maxine Waters, including after she called for people to publicly confront members of Trump’s Cabinet.

On Wednesday night, the FBI said investigators had found two packages sent to Waters “that are similar in appearance” to the others. Waters, in a statement, said she was told by police that her office “was the target of a suspicious package that has been referred to the FBI”.

Politicians called the devices attempted acts of terrorism. Trump decried the bombs in remarks at the White House on Wednesday.

But hours later at a rally in Wisconsin, he blamed the media and others for incivility in the country. On Thursday morning, less than 24 hours after CNN’s offices were evacuated, he continued on that theme with a tweet that did not directly mention the explosives but said that “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 Brennan tweeted at Trump: “Look in the mirror. Your inflammatory rhetoric, insults, lies, & encouragement of physical violence are disgraceful. Clean up your act … try to act Presidential.”

He said “your critics will not be intimidated into silence”.

Associated Press, Reuters, Agence France-Presse