What to Know
Officials are investigating up to 12 explosive packages sent to high-profile Democrats and supporters in nearly a half-dozen states
All had a return address linking to a high-profile Democratic party member
Officials briefed on the investigation say the packages were not postmarked because of their soft, irregular shape
Four more suspected mail explosives were discovered in New York City, Florida and California early Friday, one addressed to former Director of National Intelligence James Clapper, one to New Jersey Sen. Cory Booker, one to California Sen. Kamala Harris and one to businessman Tom Steyer.
The four parcels seized Friday bring the total number of suspected mail bombs to 14 and they mostly bear the same unusual hallmark -- a lumpy manila envelope with six uncancelled stamps.
But if they were sent through the mail, why weren't the stamps postmarked?
Four separate officials briefed on, or involved with, the investigation have told NBC this week that some packages were not postmarked because the soft packaging could not go through the postal machines.
In fact, the U.S. Postal Service's own website says postmarks aren't required in all cases.
"Postmarks are not required for mailings bearing a permit, meter, or precanceled stamp for postage, nor to pieces with an indicia applied by various postage evidencing systems," the USPS says on this page.
None of the bombs have detonated, no injuries have been reported and authorities said Thursday it appeared some of the devices were flawed and incapable of exploding, though it wasn't clear if they were purposefully structured that way or it was just bad construction.
Three of the packages -- two addressed to former Vice President Joe Biden in Delaware and one addressed to Robert De Niro at a building he owns in Manhattan -- were discovered in a flurry of frenzied hours Thursday.
The others came in the prior 72 hours: two were addressed to Rep. Maxine Waters of California; others were also addressed to Hillary Clinton, former President Obama, ex-Attorney General Eric Holder and former CIA Director John Brennan. The first was left at Soros' home in Westchester County on Monday.
According to sources, the explosive devices were made from PVC pipe and contained a timer (likely a digital alarm clock) to set off the detonator. The powder contained in the bombs comes from pyrotechnics. X-rays show there was likely shrapnel inside the PVC pipe, law enforcement officials say.
The return address listed on each one was the Florida office of ex-Democratic National Committee chair Rep. Debbie Wasserman Schultz. Investigators are looking into any past incidents involving her office to determine if there could be any connections to the current probe, but have said there is no suggestion she had involvement.