"These are not hoax devices", FBI Director Chris Wray said.
None of the eight packages detonated, and nobody was hurt. The official wasn't authorized to discuss an ongoing investigation publicly and spoke on condition of anonymity.
The packages discovered Thursday set off a new wave of alarm.
An FBI-led Joint Terrorism Task Force is working with federal, state and local authorities to investigate the source of the packages.
As for the other suspicious packages, nobody was harmed and the explosives never arrived at the Obamas and Clintons' homes; the packages were intercepted before they got there.
Packages have been intercepted in New York, Maryland, Florida, Delaware and Los Angeles.
Authorities said neither Mr Obama nor Ms Clinton were at risk.
The package was removed from the building about 7 a.m.
At the news conference, Cuomo disclosed that a device was sent to his Manhattan office and that it was being handled.
The packages stoked nationwide tensions and fears as voters prepared to vote November 6 to determine partisan control of Congress - a campaign both parties have described in near-apocalyptic terms. "We're all behaving very well and hopefully we can keep it that way, right?" "It has gotten so bad and hateful that it is beyond description". Trump has repeatedly criticized CNN as "fake news" and disparaged the mainstream news media as an "enemy of the people". And the way to stop that is to turn back the other way, to bring down the temperature, to end any messages about the use of violence against people we disagree with.
"Your inflammatory rhetoric, insults, lies, & encouragement of physical violence are disgraceful", Brennan wrote. CNN is known for its robust coverage of the Trump administration and for constantly provoking the ire of the president.
John Miller, the New York City Police Department's deputy commissioner for intelligence and counterterrorism, used the example of the response to the bomb that targeted Soros on Monday.
Law enforcement officials say the packages are similar manila envelopes with bubble-wrap interior packed with shards of glass, bearing six stamps and computer-printed address labels. Debbie Wasserman Schultz, D-Fla., because her name was listed as the return address on all of them.
On Tuesday, one of the law enforcement officials said the motive of the would-be Soros bomber or bombers remained unclear, adding that there had not yet been any claim of responsibility.
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Devices are pieces of pipe with white powder. There is no information to suggest she sent the packages.
The NYPD gave the all-clear about an hour after the evacuation, after investigating the packages and determining they were not a threat.
"The matter is now under federal investigation", the police said in a statement, referring questions to the FBI.
"This is someone sitting down and spending time thinking about what they're going to do to someone else". "I am very proud of some of the people that were targeted.so, if he had said anything along those lines, I certainly would have noted it".
The CNN bureau in NY received a package addressed to Brennan, who has appeared as an on-air CNN analyst, leading police to evacuate the Time Warner building in a busy Manhattan neighborhood near Central Park.
The explosive package sent to Mrs Clinton's NY home was discovered at a mail sorting facility, and did not reach the Clintons' home.
Like earlier targets, both Biden and De Niro have been sharply critical of Trump.
At a rally last weekend, Trump said Obama took Biden - often rumored as a potential 2020 Democratic presidential candidate - "off of the trash heap" when he made him vice president. "We want all sides to come together in peace and harmony".
At least five other packages were primarily sent to locations in NY and Washington, but they were also found at locations in Los Angeles and Florida.
A package containing what law enforcement believe was a small bomb was found in a mailbox outside the gates of billionaire philanthropist George Soros' home on Monday, in the NY town of Katonah.
Authorities have intercepted suspicious devices intended for former President Barack Obama and Hillary Clinton, and the Florida office of Democratic Rep. The Secret Service said neither package reached its intended recipient.
Others were sent to frequent Trump critics Waters, D-Calif., and former Obama Attorney General Eric Holder.
Asked whether the president meant to tone down his rhetoric and personal attacks, Sanders said the president would "continue to lay out the case in the differences between Democrats and Republicans" ahead of the midterm elections next month.
The Soros package also had the return address of Wasserman Schultz, a former party chairwoman, a law enforcement source and a federal source briefed on the investigation said Wednesday.