Suspected explosive devices mailed to high-profile US figures may have been sent from Florida, according to media reports.
Unnamed officials told outlets that investigators believe at least one package originated there.
But FBI Assistant Director William Sweeney would not confirm these reports during a news conference on Thursday.
Ex-Vice-President Joe Biden and actor Robert De Niro were the latest targets.
Similar packages have also been sent to former President Barack Obama and former Secretary of State Hillary Clinton, among others.
Investigators are eyeing southern Florida in particular, the New York Times reported, saying it had spoken to two people familiar with the matter.
- What we know about US mail bomb threats
At Thursday’s news conference, officials said they could not give any details because it could affect the ongoing investigation.
However Mr Sweeney did confirm that the white powder found in one of the packages – delivered to CNN and addressed to former CIA director John Brennan – “did not present a biological threat”.
Mr Sweeney said thorough examinations of all the packages are under way at the FBI laboratory in Quantico, Virginia, outside Washington DC.
New York Police Commissioner James O’Neill could not confirm whether all the devices were intended to explode, but he said officials “are treating them as suspected explosive devices”.
“This has to be taken with the utmost seriousness,” he said. “As far as a hoax device? We’re not treating it that way.”
Mr O’Neill added that authorities “do not know at this time” whether there are additional devices in the mail. He said police are working with postal service officials “to make sure if there are any out there, we identify them quickly”.
On Thursday morning, the FBI said Mr Biden received two suspect packages and a similar package was found at a New York restaurant owned by Mr De Niro.
How did the bomb threat unfold?
The series of bomb alerts began on Monday, when a suspected device found in the post box of billionaire businessman George Soros, a major Democratic Party donor.
Other devices were sent to the following individuals, according to the FBI:
- Former Secretary of State Hillary Clinton
- Former President Barack Obama
- Former Vice-President Joe Biden
- Former CIA Director John Brennan, care of CNN
- Former Attorney General Eric Holder
- California Democratic Congresswoman Maxine Waters
- Actor Robert De Niro, via his restaurant
None of the devices went off.
What happened with De Niro?
A suspected explosive device was found at a building in Manhattan early on Thursday, local time. It is home to Mr De Niro’s production company and Tribeca Grill restaurant.
New York City Mayor Bill de Blasio praised the “really quick-witted work of a security guard” who alerted authorities.
According to the Associated Press, the security guard was off work on Thursday and had seen an image in a news report of the packages others had received.
That individual then recalled spotting something similar in the building’s mailroom and immediately called authorities who removed the device around 06:30 (10:30 GMT), AP reports.
CNN’s New York office was evacuated on Wednesday morning, after the package addressed to Mr Brennan was found in its mailroom.
Why is this so political?
Every person who has has been sent package is known to be a critic of President Donald Trump.
The attempted bombings also come just under two weeks before the mid-term elections, with US politics highly polarised.
On Wednesday, President Trump responded to the unfolding news of the suspect packages by calling for more civility in public life.
“Those engaged in the political arena must stop treating political opponents as being morally defective,” he said at a rally in Wisconsin.
“No-one should carelessly compare political opponents to historic villains, which is done often.”
The president made no specific reference to the intended recipients of the packages.
His critics have called his remarks hypocritical, as he often uses vicious language against his opponents and the press.
Former CIA Director John Brennan, who was targeted at the CNN office on Wednesday, tweeted to Mr Trump on Thursday: “Stop blaming others. Look in the mirror. Your inflammatory rhetoric, insults, lies, & encouragement of physical violence are disgraceful.”
However, conservatives say Democrats are to blame for the state of political discourse in the US.
They say Democrats, including those who were targeted in the bomb scare, have encouraged “angry mob” behaviour.