At least some of the devices found in the wave of suspected pipe bombs mailed to prominent political and media figures had no possibility of exploding — because they contained flaws in the way they were made, several investigators told NBC News on Thursday, based on early analysis.

In a few cases, the flaws were substantial, the investigators said. In others, the defects were more subtle. But there's still no way to know at this point whether the deficiencies were intentional — to make convincing hoaxes — or simply the result of bad design or construction, the sources said.

The investigation's analysis is far from complete, as some devices have yet to be analyzed in any kind of detail.

And officials continued to caution that anyone who gets one of the packages should treat it as dangerous.

Pressed at a news conference on Thursday afternoon as to whether the devices were supposed to go off or not, New York Police Commissioner James P. O'Neill would say only: "We are treating them as suspected explosive devices."

Law enforcement officials close to the investigation told NBC News on Wednesday that the bombs were made from PVC pipe with a digital timer attached to set off the detonator. Each bomb also contained powder from a low explosive or a pyrotechnic, as in fireworks, they said.

X-rays showed that pieces of shrapnel were likely to be inside the PVC pipes, the officials said.

The list of mail bombing targets grew on Thursday to include the outspoken anti-Trump actor Robert De Niro and former Vice President Joe Biden, as well as former Secretary of State Hillary Clinton, former President Barack Obama, former CIA Director John Brennan, the billionaire George Soros, former Attorney General Eric Holder and Rep. Maxine Waters, D-Calif.

A suspected explosive device received at the CNN bureau in New York. NBC News

The device sent to Brennan was addressed to the CNN offices in New York — even though he is a national security analyst for NBC News — prompting an hourslong evacuation of the building.

A similar device was found in Soros' mailbox on Monday. All of the packages were intercepted before reaching their targets.

The FBI has sent the devices to its laboratory in Quantico, Virginia, for analysis.

Two senior law enforcement officials, along with another official briefed on the investigation, said they were looking into whether some of the packages were mailed from Florida. A senior official said investigators were specifically looking at a mail facility north of Miami, in Opa-Locka.

An X-ray image of one of the pipe bombs addressed to CNN and prominent critics of President Donald Trump provided by law enforcement officials briefed on the case. NBC News

MSNBC senior law enforcement analyst Bill Bratton, who led the police departments in Boston, New York and Los Angeles, told MSNBC on Thursday that the FBI had a number of devices that it would be "in a position to disassemble to determine their functionality."

"Were they ever intended to actually function, and if so, how, in terms of how were they supposed to be detonated?" he asked.

"If not, well, then, that changes the focus from trying to cause bodily harm to effectively creating fear," he said.

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Bratton said there had been some discussion about a "timing device" on the explosives, which raised further questions.

"These things are being found over the space of several days, so what was the timer set for, with no guarantee when it would, in fact, be opened or where it would be at any given time?" he said.

Bratton added that the sheer number of devices would help investigators.

"The building back of the investigation is going to be assisted tremendously by the fact that there are so many of these things, 10 of them," he added. "And there's the potential for a slip-up."