CNN
Now playing
04:22
Who got what, when: A timeline of when bombs were found
mail bomb suspect attorney
CNN
mail bomb suspect attorney
Now playing
03:43
Attorney: Sayoc found a father in Trump
CNN
Now playing
02:42
Trump's rhetoric on suspicious packages
rochelle ritchie
CNN
rochelle ritchie
Now playing
02:45
Woman warned Twitter about bomb suspect
CNN
Now playing
02:23
Trump: Media using mail bombs for political gain
sayoc perp walk
WPLG
sayoc perp walk
Now playing
00:30
See suspect in police custody
Broward County Sheriff
Now playing
01:09
Listen to police audio of Cesar Sayoc arrest
Debra Gureghian store manager EBOF
CNN
Debra Gureghian store manager EBOF
Now playing
01:50
Suspect's manager: He told me to burn in hell
Christopher Wray explosives presser 10262018
CNN
Christopher Wray explosives presser 10262018
Now playing
01:16
Wray: 13 bombs sent are not hoax devices
WPLG/Broward County Sheriff's Office
Now playing
01:48
See pro-Trump stickers on bomb suspect's van
Broward County Sheriff's Office
Now playing
03:37
See suspect's disturbing social media posts
Tom Steyer, co-founder of NextGen Climate Action Committee, attends a "Need To Impeach" event in Detroit, Michigan, U.S., on Monday, Aug. 13, 2018. Steyer plans to spend an additional $10 million on helping Democrats take control of the House by attempting to drive up turnout among infrequent voters motivated by the idea of impeaching President Donald Trump.
Anthony Lanzilote/Bloomberg/Getty Images
Tom Steyer, co-founder of NextGen Climate Action Committee, attends a "Need To Impeach" event in Detroit, Michigan, U.S., on Monday, Aug. 13, 2018. Steyer plans to spend an additional $10 million on helping Democrats take control of the House by attempting to drive up turnout among infrequent voters motivated by the idea of impeaching President Donald Trump.
Now playing
01:42
Package addressed to Tom Steyer found
REFILE-CORRECTING TYPO  A member of the New York Police Department bomb squad is pictured outside the Time Warner Center in the Manhattan borough of New York City after a suspicious package was found inside the CNN Headquarters in New York, U.S., October 24, 2018.
Kevin Coombs/Reuters
REFILE-CORRECTING TYPO A member of the New York Police Department bomb squad is pictured outside the Time Warner Center in the Manhattan borough of New York City after a suspicious package was found inside the CNN Headquarters in New York, U.S., October 24, 2018.
Now playing
02:24
What was inside the bombs?
WCBS
Now playing
02:01
New suspicious packages addressed to Booker, Clapper
CNN
Now playing
02:16
Sanders: 1st thing CNN did was blame Trump
(CNN) —  

A starting point for investigators trying to trace the bombs sent to Democratic figures and CNN this week: the postmark.

But in some cases, it appears to be a missing clue.

The postmark typically indicates when and where the mail was sent from and is marked over the stamps – “canceling” them, and preventing the stamps from being reused. Unlike stamps, most of which cannot be traced, the impression left by a postmark or mail meter is distinctive and can help investigators focus their search.

A postmark is visible in a photo of at least one of the packages seen by CNN. A postmark is not visible in photos of several other packages shared with CNN – only adding to the questions about their origins.

The lack of a postmark is very rare, according to Nancy Pope of the National Postal Museum.

A postal inspector who spoke to CNN on the condition of anonymity indicated it would be very unusual for a piece of mail to leave the facility where it was dropped off for delivery without such a mark.

That could indicate that a sender or senders were working in multiple diverse geographical areas, including New York, Washington, Florida and California.

Another source told CNN it is possible some of the stamps may not have been canceled because of the package’s soft sides and odd shape, which would make it difficult to stamp.

The US Postal Service’s website also says mail may not be postmarked if it was processed through a meter or if the postage was paid by permit or precanceled stamp. None of those appear to apply to this situation.

Adding to the mystery are post office markings on one of two packages sent to former Vice President Joe Biden indicating the stamps on the package did not cover the cost of the delivery, and additional postage is due.

That suggests the package was weighed at some point during the shipping process, a former postal inspector told CNN.

It was unclear from the photo whether that package had been postmarked.

Postmarks can be applied by machine and by hand, depending on how a piece of mail is sent, according to the Postal Service. When mail is run through an automated system at a processing distribution center, the postmark is applied, and the mailed item is screened by a biohazard detection system.

It was not clear why postmarks were not visible on several of the packages, and the US Postal Inspection Service provided a statement to CNN that did not address the question.

“The U.S. Postal Service has developed a comprehensive approach to protecting the mailsystem by utilizing a targeted strategy of specialized technology, screening protocols and employee training,” read the statement.

The statement noted its Dangerous Mail Investigations Program includes law enforcement officers who “are trained to recognize the common characteristics of suspicious mail and are highly proficient in the use of state-of-the-art equipment to include portable X-ray machines.”