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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.”