A manhunt was underway on Thursday after high-profile Democrats and CNN were targeted by package bombs, less than two weeks ahead of elections that could alter the US political landscape.
None of the seven devices went off and no injuries were reported, but some top Democrats called the threats a symptom of a coarsening brand of political rhetoric promoted by President Donald Trump, who also condemned the acts.
All the targets are frequently maligned by right-wing critics, including former US President Barack Obama, former Secretary of State and presidential candidate Hillary Clinton and Obama's Attorney General Eric Holder.
Former CIA Director John Brennan, prominent Democratic Party donor George Soros and California Representative Maxine Waters, an outspoken critic of Trump, were also targeted.
On Thursday morning, police were investigating a suspicious package at a building in the Tribeca section of New York City that houses a restaurant owned by actor Robert De Niro, according to NBC's New York affiliate.
The bomb packages were sent as the nation prepares for Nov. 6 elections that will decide whether Democrats take control of one or both houses of Congress from Republicans and deny Trump the majority his party now holds in both.
There was no immediate claim of responsibility.
An FBI-led Joint Terrorism Task Force of federal, state and local law enforcement agencies "will continue to work to identity and arrest whosoever is responsible for sending these packages," FBI Director Christopher Wray said in a statement.
Several politicians, including Senate Majority Leader Mitch McConnell and New York Mayor Bill de Blasio, described it as an act of terrorism.
Investigators were trying to track down another suspicious parcel they believe was addressed to Obama's vice president, Joe Biden, a federal official told Reuters late on Wednesday.
The CNN bureau in New York 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 package contained an envelope of white powder that experts were analyzing, Police Commissioner James O'Neill said.
Trump told a political rally in Wisconsin on Wednesday his government would conduct "an aggressive investigation."
"Any acts or threats of political violence are an attack on our democracy itself," Trump said. "We want all sides to come together in peace and harmony."
He also said the media had a responsibility "to stop the endless hostility and constant negative and oftentimes false attacks and stories."
Last week, Trump, who has joined other Republicans in accusing Democrats of encouraging "mob" tactics, heaped praise on a Montana congressman who assaulted a reporter in 2017.
At an event in Texas on Wednesday, Brennan faulted Trump for frequent inflammatory rhetoric.
The first package, which turned up on Monday, was addressed to Soros, the billionaire financier and advocate of liberal, open-border values who is a frequent target of right-wing conspiracy theories.
The parcel intended for Holder ended up rerouted to the return address printed on all the packages - the Florida office of U.S. Representative Debbie Wasserman Schultz, who formerly chaired the Democratic National Committee, according to the Federal Bureau of Investigation.
The FBI said late on Wednesday it had confirmed that two additional packages, similar in appearance to the others, were addressed to Waters.
An FBI statement said the other packages consisted of a manila envelope with a bubble-wrap interior containing "potentially destructive devices". Each bore a computer-printed address label and six "Forever" postage stamps.
Other officials said the devices contained in the envelopes were all similar to one found in the mailbox of the Soros home and later detonated by police. At least one bomb was packed with shards of glass, one federal source said.