(CNN) Law enforcement officials from New York to Florida are examining clues to find the person or persons who sent 10 packages containing what appear to be explosive materials to prominent Democrats, critics of President Donald Trump, and to CNN's New York offices as authorities classify the incidents as domestic terrorism.

Authorities believe several of the packages went through the Opa-locka, Florida, processing and distribution center, according to two law enforcement officials familiar with the investigation.

The devices mark one of the most serious attempts to stoke fear and terror in the US in a period that has already seen a number of violent political attacks. The apparent bombs, while rudimentary in design, according to law enforcement officials, targeted two former US presidents, a former vice president and potential presidential contender, and numerous outspoken critics of the Trump administration.

Three new packages were detected Thursday -- two intended for former Vice President Joe Biden and one for actor-director Robert DeNiro -- each bearing the same markings as the seven packages detected earlier this week.

Later Thursday, parts of Time Warner Center -- the building that houses CNN's bureau in New York City -- were evacuated after two suspicious packages were found in the building. The New York Police Department's bomb squad declared the scene to be all clear just after 8:30 p.m. Two small boxes had been left unattended and the packages didn't match the description of those sent to CNN and political figures.

As authorities examine clues, they are also finding more questions to answer.

One key question authorities are puzzling over is why none of the apparent bombs detonated, a second law enforcement source said, raising questions about the skill and motive of the bombmaker.

This person said either the bombmaker was good enough to ensure none would go off and never intended them to explode or it was poorly constructed. Authorities consider the bombs to be potentially destructive because of the presence of what is believed to be pyrotechnic powder. Though it appears they were handled through the postal system -- and thus jostled and moved -- without triggering any explosion.

William Sweeney, FBI special agent in charge of the New York field office, said Thursday afternoon that the powder discovered with the package sent to CNN did not pose a biological threat but another official said it was undergoing testing.

The FBI's counterterrorism team is leading the investigation and has classified it as a domestic terror matter, the law enforcement official said. That classification is based on the absence of other information and could change.

The Secret Service, US Postal Inspector Service and ATF are also working on the investigation with various state and federal offices. Officials said it is possible additional packages could have been mailed to other locations and not yet discovered.

NYPD and FBI officials stressed Thursday that the investigation was still in its early stages and they continued to seek tips from the public.

"Everything is a clue," a law enforcement official said.

Fingerprints and evidence

The bombs were packaged in manila envelopes with bubble-wrap lining, according to the FBI. They each bore computer-printed address labels and six American flag stamps.

All 10 packages went through the US mail system, according to multiple law enforcement officials familiar with the investigation.

The package addressed to John Brennan at CNN's New York office was delivered by a private courier company who delivers the mail to CNN on a daily basis, one of the law enforcement officials said. It is not uncommon for large companies or public figures to hire outside companies to pick up the mail at the post office, scan it off site and deliver it to the intended recipient, the official added.

Previously officials had said the package delivered to George Soros was placed in his mailbox. The law enforcement source said Thursday that Soros utilizes a company to filter his mail.

Investigators will be looking for fingerprints on the package, on the back of any tape that may have been used, and on the stamps and labels. If the seals were licked, investigators may be able to pull DNA from the saliva, officials say. They will also look for any hairs or other materials that could help identify the sender or location.

Investigators who traced a string of package bombs in Texas that killed two people and wounded five in March used those same tools to pull DNA from the packages.

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In addition, investigators used signals from cell phone towers to help narrow down the potential suspects in the area at the time the packages were dropped off, according to a law enforcement official. They relied on closed circuit cameras to narrow the field further, the official said. It took investigators 18 days to track down Mark Anthony Conditt, who killed himself with one of his own explosive devices.

Those tactics are likely to be deployed as authorities trace couriers and any individual who may hand delivered the packages.

Homeland Security Secretary Kirstjen Nielsen was asked in an interview by Fox News about indications the packages came from Florida. She said the FBI is tracking down all leads, noting "as you know some of the packages went through the mail. They originated some of them from Florida. So that was an obvious lead to pursue."

Bombs transported to Quantico

All of the bombs are being transported to the FBI lab in Quantico, Virginia, for analysis, officials say.

Other information on the packages will also be rich investigative lines. If the packages were scanned by the US postal system, investigators may be able to determine which machines scanned the documents and when. However, unlike FedEx or other private mail delivery services, the US mail system is not set up to track every single piece of mail.

Authorities believe several of the packages went through the Opa-locka processing and distribution center. According to a US Postal Service employee, this facility handles mail that is incoming and outgoing from south Florida.

Philip Bartlett of the US Postal Inspection Service told reporters that authorities are not yet able to say where the packages were mailed from.

The Miami-Dade Police says it is assisting the investigation at the Florida mail facility, the department tweeted

At the lab the bombs themselves will be dissected. According to a law enforcement official, the bombs so far appear to show the presence of a sulfur substance, which would be the ingredient that was intended to explode. Authorities will examine the components of the bomb and look for clues as to where they may have been purchased and any similarities or differences between the individual packages. Pipe bombs are inherently unstable devices and could be set off simply by handling them.

Outside experts have pointed to the lack of a triggering mechanism, suggesting it was never meant to explode. The device includes very common components, making it more difficult to get clues from the signature of the bomb. But the components could still provide clues -- like the clock and the tape used.