Authorities found suspicious packages addressed to former vice president Joe Biden and the actor Robert De Niro, officials said Thursday, setting off a new wave of alarm amid a sprawling investigation into pipe bombs mailed to prominent political figures and critics of President Donald Trump.

The new packages, which resemble those sent to figures including former president Barack Obama and former secretary of state Hillary Clinton, were located in Delaware and New York. Like the others, they were intercepted before reaching their intended targets. The devices found so far, which officials have described as pipe bombs, have put many in the country on edge, and authorities have begun an intensive effort to find other explosives that may still be out there.

Investigators found a package addressed to Biden in a Delaware mail facility that was like the other pipe bombs found this week, according to a law enforcement official. 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.

In a statement Thursday morning, the FBI declined to confirm that a package addressed to Biden was found, instead saying only that local and state police, along with U.S. 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 law enforcement officials. This package was discovered around 5 a.m. by security personnel working for De Niro's film company, Tribeca Productions, according to police. A police spokesman said the parcel "was similar to explosive packages that had been publicized."

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 Hillary Clinton, Barack Obama and others, a law enforcement official said.

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

New York Mayor Bill de Blasio (D) 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."

The hunt for a serial mail bomber began late Tuesday when Secret Service personnel conducting standard mail screenings found a pipe bomb inside a plain manila envelope with a bubble-wrapped interior addressed to Clinton, the former secretary of state. A similarly packaged bomb was found hours later in mail addressed to Obama.

Neither bomb got close to its intended target, and neither contained a written message, according to law enforcement officials, but the twin discoveries led to other undetonated devices being found elsewhere, including in the mailroom at CNN's New York headquarters and a district office of Rep. Debbie Wasserman Schultz, D-Fla.

As more devices were discovered, a disturbing pattern emerged — the bomber or bombers appeared to be targeting prominent figures on the political left.

The first such package was discovered Monday at the New York home of George Soros, a billionaire activist known to fund pro-democracy and liberal political groups, many of whom have been targets of President Donald Trump's incendiary rhetoric.

The first such package was discovered Monday at the New York home of George Soros, a billionaire activist known to fund pro-democracy and liberal political groups.

The package addressed to De Niro came after he had also publicly tangled with Trump.

"(Expletive) Trump," he said then. "It's no longer down with Trump," he said, "but (expletive) Trump."

Days later, Trump fired back on Twitter "Robert De Niro, a very Low IQ individual, has received too many shots to the head by real boxers in movies. I watched him last night and truly believe he may be "punch-drunk.' I guess he doesn't..."

Majority Leader Mitch McConnell, R-Ky., and other officials described the devices discovered this week as attempted acts of terrorism, and the authorities cautioned that the number of bombs — and intended victims — could grow. FBI Director Christopher A. Wray issued an appeal for the public’s help to catch the bomber and warned people not to touch suspicious packages.

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.

The FBI, the Secret Service, the NYPD and other agencies kicked into high gear Wednesday morning trying to determine who was behind the bombs — and how many more may be en route to intended victims. At least two of the devices appeared to have been hand-delivered, according to law enforcement officials.

"We will not rest until we stop these hazardous devices from being mailed and bring the individual or individuals to justice," said C. Bryan Paarmann, an FBI counterterrorism official. The devices have been sent to the FBI's lab in Quantico, Virginia, for further analysis.

Officials said the packages used as a return address the office of Wasserman Schultz, who chaired the Democratic National Committee during part of the 2016 campaign. One such package used an incorrect address for former attorney general Eric H. Holder Jr., and it was "returned" to her office in Sunrise, Florida, where it was intercepted, according to officials. Authorities do not believe she had anything to do with the packages and think she was a potential victim. A spokesman for Holder declined to comment.

The pipe bomb addressed to former Obama administration CIA director John Brennan was found in the mail at CNN's headquarters in New York. Since leaving the government, Brennan has been an outspoken critic of President Trump; he is an on-air analyst for NBC News and MSNBC, rather than CNN.

On Wednesday night, the FBI issued a statement saying investigators found "two additional packages, both addressed to Rep. Maxine Waters, that are similar in appearance" to the other five.

In a statement, Waters, D-Calif., said she had been told by police that her office "was the target of a suspicious package that has been referred to the FBI. I am appreciative of the law enforcement entities who intercepted the package and are investigating this matter. I unequivocally condemn any and all acts of violence and terror."

The packages came with computer-printed address labels, six Forever stamps and return addresses with Wasserman Schultz's name misspelled, officials said.

Amid the frenetic pace of the investigation, some alarms turned out to be false. A suspicious package sent to New York Gov. Andrew Cuomo, a Democrat, was in fact a thumb drive, according to the NYPD. Authorities also searched an office building in San Diego that houses a newspaper before concluding there was nothing dangerous there.

The known recipients of the bombs are all frequent targets of conservative politicians, including Trump. Speaking at the White House, the president decried the bombs and pledged that the federal government would bring those responsible to justice.

"In these times, we have to unify," he said. "We have to come together and send one very clear, strong and unmistakable message that acts or threats of political violence of any kind have no place in the United States of America."

But hours later, at a rally in Wisconsin, Trump blamed the media and others for incivility, even as he refrained from acknowledging his own inflammatory rhetoric.

Speaking in Florida on Wednesday, Clinton said her family was "fine, thanks to the men and women of the Secret Service who intercepted the package addressed to us long before it made its way to our home."

She also spoke to the anger pulsing through American politics: “It is a troubling time, isn’t it? It is a time of deep divisions, and we have to do everything we can to bring our country together.”