Investigators are expressing confidence they will identify who is responsible for sending 10 pipe bombs to prominent Democrats and critics of U.S. President Donald Trump.
"We will identify and arrest the person or people responsible for these acts," New York City Police Commissioner James O'Neill told reporters Thursday afternoon.
All the "suspected explosive devices" were being taken to the Federal Bureau of Investigation's laboratory at the U.S. Marine Corps base in Quantico, Virginia, said O'Neill.
"We are discovering things by the hour," he added at a news conference where other officials also said tips from the public were providing leads for investigators.
FBI Assistant Director William Sweeney, who is in charge of the bureau's New York field office, said "the investigation is still in its early stages," so he would not provide specific answers to the media about possible suspects or whether the pipe bombs were properly wired to explode.
Investigators are specifically looking at the southern part of the state of Florida as a source of some of the packages, according to media reports.
Trump is largely blaming the media for the angry political atmosphere in America that critics contend has led to what is being regarded as a wide-scale assassination attempt.
"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," Trump said on Twitter on Thursday morning.
New packages meant for Biden, De Niro
Trump's tweet came as suspected explosive devices addressed to actor Robert De Niro and former Vice President Joe Biden were found Thursday.
Police in New York said one package was sent to a production company owned by De Niro and that it is similar to those sent to other prominent Democrats, including former president Barack Obama and former secretary of state Hillary Clinton. The New York City Police Department's bomb squad transported the suspected bomb in a special truck out of the Tribeca neighborhood Thursday.
The Federal Bureau of Investigation also confirmed two packages addressed to Biden were found in Delaware.
WATCH: Bomb Expert: Pipe Bomb Origins Are Easy to Detect
Other packages have been addressed to a former U.S. attorney general, two Democratic Party members of Congress and former Central Intelligence Director John Brennan.
Brennan, now a commentator for MSNBC network and NBC News, responded to Trump's tweet Thursday, tweeting "Stop blaming others. Look in the mirror. Your inflammatory rhetoric, insults, lies & encouragement of physical violence are disgraceful."
What police termed a live explosive device that arrived in an envelope addressed to Brennan at CNN's New York bureau prompted the evacuation of the Time Warner Center for hours Wednesday.
Pressed by reporters Thursday on whether there could be a link between Trump insulting political opponents and the dispatch of the bombs, Sanders replied, "the president is certainly not responsible for sending suspicious packages to someone no more than [Democratic U.S. Senator] Bernie Sanders was responsible for a supporter of his shooting up a baseball field practice [by Republican lawmakers] last year."
Trump, lawmakers weigh in
Trump said at a rally late Wednesday that targeting government officials by sending them explosives "is an attack on our democracy itself."
Throughout the day Wednesday, leaders from both the major parties called for a return to civility in the political arena, a theme Trump picked up on at the rally.
"No one should carelessly compare political opponents to historical villains, which is done often and all the time. It's got to stop," said the president. "We should not mob people in public spaces or destroy public property. There is one way to settle our disagreements — it's called peacefully, at the ballot box. That's what we want."
House Minority Leader Nancy Pelosi and Senate Minority Leader Chuck Schumer, the top two Democrats in Congress, issued a statement Wednesday afternoon, accusing Trump of fanning the flames of political unrest.
"I think this is an act of political terrorism," Congressman Rodney Davis, a Republican from the state of Illinois, said on CNN early Thursday.
The Secret Service says the package addressed to Clinton was discovered late Tuesday, intercepted at a mail screening facility near her home in a New York suburb where she lives with her husband, former President Bill Clinton.
A separate package addressed to Obama, according to the Secret Service, was intercepted at a screening facility at Joint Base Anacostia-Bolling, a 365-hectare military facility in Washington.
California Congresswoman Maxine Waters was sent two suspect packages, one intended for her office in the nation's capital and the other for her home district office. The first was intercepted at a congressional mail sorting center in the state of Maryland, and the second discovered by postal inspectors at the Los Angeles Central Mail Sorting Facility.
The first in the series of explosive devices was found Monday in a mailbox outside the New York home of billionaire philanthropist George Soros, a major donor to Democratic candidates.
Ken Bredemeier and Masood Farivar contributed to this report.