(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.

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