The Boston Police Department has launched an investigation Monday evening after a United States Postal Service carrier discovered charred mail inside one of the agency's mailboxes.

Police said the incident occurred during a 4 pm pickup from a mailbox located on Boylston Street that was last emptied on Saturday with no signs of damage. Authorities noted that a U.S. postal inspector took custody of both the damaged mail and the mailbox.  Postal inspectors investigate criminal allegations or incidents involving the sending or receiving of mail via the USPS.

A USPS spokesperson did not return FOX News' request for comment by the time of publication.

BOSTON MAN ARRESTED IN CONNECTION TO BALLOT DROP BOX FIRE INVESTIGATED BY THE FDBI

The incident comes on the heels of a fire started at a nearby ballot drop off box in Copley Square, near the main campus of the Boston Public Library.

Police arrested 39-year-old Worldy Armand in connection to that incident. Armand is expected to be arraigned in Boston Municipal Court on the charge of willful and malicious burning.

It was not immediately clear whether the two incidents were connected. 

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Anyone with additional information related to the investigation should contact the Boston Fire Investigation Unit at (617) 343-3324.

This is a developing story. Check back for updates.