(CNN) Authorities have now found two packages addressed to former Vice President Joe Biden that are similar in appearance and contain potential explosive devices that resemble the others sent earlier this week to prominent critics of President Donald Trump and CNN, a law enforcement official said Thursday.

The two packages, discovered at Delaware postal facilities, were removed late Thursday morning, law enforcement officials said.

Earlier Thursday, a suspicious package addressed to actor-director Robert De Niro was reported at the Manhattan building where his production company is based, and its marking and contents appear similar to the other pipe bomb packages , two law enforcement sources said.

The FBI said Thursday morning that the three packages are "similar in appearance to the others."

A manhunt is underway for the perpetrator behind the packages. Law enforcement authorities are treating the series of bombs as a domestic terror matter and are advising the public to remain vigilant.

The motive is unknown, but the recipients are all prominent targets of right-wing criticism and, in many cases, of Trump himself. The package found at CNN's New York bureau in the Time Warner Center was addressed to former CIA Director John Brennan, city and local law enforcement officials said.

Devices sent to former President Barack Obama, Hillary Clinton, former Attorney General Eric Holder, California Democratic Rep. Maxine Waters and Democratic donor George Soros showed the presence of a sulfur substance, which could have exploded, a law enforcement official said.

Examining the devices

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The devices are believed to be pipe bombs, inherently unstable, and at risk of being set off just by handling. An FBI bomb truck, containing the device sent to Brennan, left New York City Thursday morning for the FBI lab at Quantico, Virginia, a law enforcement official familiar with the investigation said.

A suspicious powder from the package was tested and initially deemed not to be anthrax or a known agent, the source said, adding that additional testing will continue over the coming days.

Authorities are not giving credence to the reports of an image of the ISIS flag on the device, the source said, meaning they do not believe it's international terrorism. The image appears to be a spoof of an ISIS flag that has been widely circulated on the Internet, an image that essentially parodies the ISIS flag, the source added.

Authorities are questioning why none of the bombs went off.

The FBI called them "potentially destructive devices," so either the bombmaker was good enough to ensure none would go off and never intended them to explode, or they were poorly constructed.

However, the presence of what is believed to be pyrotechnic powder is one reason why authorities consider them to be potentially destructive, though it appears they were handled through the postal system -- where they were jostled and moved -- without any explosion.

Outside experts have pointed to the lack of a triggering mechanism, suggesting they were never meant to explode.

The construct 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.

Investigators believe some of the packages may have originated in Florida, according to one law enforcement official.

A second law enforcement official said federal investigators are in Florida investigating leads.