The FBI has searched a mail facility near Miami, as investigators try to find who is behind a series of suspected explosive packages.
Devices have been sent to eight high-profile people, including ex-President Barack Obama and actor Robert De Niro.
Unnamed officials told US media that investigators believe at least one was sent from Florida.
The FBI have not released any details of their investigation.
Similar packages have also been sent to former Vice President Joe Biden and former Secretary of State Hillary Clinton, among others.
The postal facility searched is in Opa-locka, where security footage was being scrutinised. A bomb squad and canine unit accompanied federal officials, Miami-Dade County Police said.
On Thursday evening, the Time Warner Center in New York was evacuated after reports of a suspicious package, but it was later declared a false alarm. The building is home to news network CNN, which received one of the devices by post the previous day.
Earlier, FBI Assistant Director William Sweeney confirmed that the white powder found in the package sent to CNN “did not present a biological threat”.
Mr Sweeney said thorough examinations of all the packages were 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 were additional devices in the mail. He said police were 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.