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.
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
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.
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.
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.
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.