Bart Jansen and Kevin Johnson

USA TODAY

Federal investigators are increasingly focusing on Florida as a transit point for a number of the 10 suspected explosive devices recovered so far in a chilling terror campaign aimed at top Democratic Party leaders and supporters, a law enforcement official said Thursday.

The official, who is not authorized to comment publicly, said among the locations that have drawn investigative interest has been the postal distribution center in Opa Locka, Florida, where several of the packages may have passed through.

Miami-Dade police bomb-squad and K-9 officers were called out to the center Thursday evening as federal authorities continued their investigation at the facility.

Attention turned to Florida on Thursday largely because the device addressed to former Attorney General Eric Holder was recovered when it was routed back to the return address of Rep. Debbie Wasserman Schultz, D-Fla., in Sunrise, according to two law enforcement officials. Schultz has nothing to do with the mail campaign.

The two officials, who are not authorized to comment publicly, said authorities had not identified a suspect.

Three new potentially explosive devices found Thursday were nearly identical in design to the seven seized earlier in the week, a law enforcement official said Thursday. One of the latest was sent to actor Robert De Niro at offices in New York, and two were sent to former Vice President Joe Biden and intercepted at postal facilities in Delaware.

The official, who is not authorized to comment publicly, said the devices were assembled with pieces of plastic PVC pipe containing black powder and shards of shrapnel that appeared to be glass. The pipes were wrapped in tape and a timing-like mechanism was attached.

New York Police Department Commissioner James O’Neill said Thursday that the devices were not being treated as hoaxes, but he stopped short of calling them “live” devices.

“I would say it’s a suspected explosive device,” O’Neill told reporters at an afternoon news conference. “We are treating them as suspected explosive devices.”

The powder found in packaging in New York wasn’t a biological weapon, but further testing is being done, O’Neill said. The devices were being examined at the FBI lab in Quantico, Virginia.

“We are discovering things by the hour," he said.

FBI Assistant Director Bill Sweeney said the investigation is nationwide and still in its early stages. He declined to provide details about the devices to avoid disclosing significant information.

“It does remain possible that further packages have been or could be mailed,” he said. “These devices should be considered dangerous.”

Kevin Barry, a former member of New York Police Department's bomb squad and a former director of the International Association of Bomb Technicians and Investigators, expected the FBI analyses to be focused on both the design and recovery of physical evidence.

Testing would gauge the functionality of the devices, check the type of pipe used and examine the powder to determine whether it is high explosive, low explosive or a chemical substance intended to be dispersed when the devices were triggered, Barry said.

If the tests found a fingerprint on the devices or their packaging, the FBI would run that evidence through a criminal database seeking a match. Investigators would likely be able to identify a suspect within days from a matching fingerprint, Barry said.

Philip Bartlett, the U.S. Postal Inspection Service's inspector in charge of the New York division, said 600,000 postal employees have been scanning packages for more devices.

“We have found nothing in the last eight hours,” he said.

Although the two devices addressed to Biden were recovered Thursday, investigators were chasing the possibility late Monday that Biden could be targeted, sorting through the public mail streams in search of the familiar heavily stamped manila envelope. Biden is no longer under the protection of the U.S. Secret Service.

Other packages found this week were sent to former officials and high-profile people, including former Secretary of State Hillary Clinton, former President Barack Obama and former CIA Director John Brennan at CNN.

As the FBI, Secret Service and local police track down who built and mailed the devices, the concern remains how many more might be out there – and whether more disruption is planned.

If all the explosives were sent to one party or political affiliation, that could provide clues about the sender, said Anthony Roman, president of Roman & Associates, a risk-management and commercial-investigation firm in Uniondale, N.Y.

More:More bomb-like devices found, 1 near Robert De Niro's offices in NYC and 2 addressed to Joe Biden in Delaware

More:What we know about the 10 potential pipe bombs sent to De Niro, Biden, Clinton, Obama

The attacks also could be a precursor to bombings of public areas such as subways or iconic buildings – or to disruptions at polling places for the Nov. 6 election, Roman said.

“For police and federal officials and intelligence authorities not to consider that would be irresponsible,” Roman said. “I’m sure that is a scenario that is being considered and will be protected against.”

New York City police were stationed outside media locations and offices of elected officials, New York Mayor Bill de Blasio said.

“You’re going to see a lot of police presence," he said.

Homeland Security Secretary Kirstjen Nielsen said her department and the Secret Service are coordinating closely with the FBI and have heightened the security at federal facilities around the country.

"I condemn these cowardly acts in the strongest possible terms," Nielsen said. "Americans will not tolerate these types of threats, and we will not be intimidated.”

The U.S. Postal Inspection Service said in a statement Thursday that the agency has a comprehensive approach to protecting the mail through specialized technology, screening and employee training.

Inspectors in the service's Dangerous Mail Investigations Program "are trained to recognize the common characteristics of suspicious mail and are highly proficient in the use of state-of-the-art equipment to include portable X-ray machines," the agency said. "Any reports of suspicious mailings are taken very seriously, as they may impact the safety of postal employees and disrupt the processing of mail."

The service declined to publicly describe its investigative procedures or operational protocols.

The American Postal Workers Union said its members were "deeply concerned for their own well being and that of the public we serve" that explosives were sent through the mail.

The Postal Service is in the process of nationwide safety talks covering how to recognize suspicious packages and what protocols to follow.

Wherever packages were mailed, authorities will review video surveillance to try to identify the senders, experts said.

“Then it goes back to old-fashioned detective work," Roman said.

Contributing: Kevin McCoy