Montgomery County police are investigating a string of mailbox robberies, where perpetrators are stealing checks intended for bills and cashing them.

Police said the thieves are stealing the mail, removing the checks and then altering the checks to cash them at area banks.

The thefts have been occurring throughout the county since November 2012, police said.

To protect against theft, police are advising residents to drop important outgoing mail in secure U.S. Postal Service mailboxes or at post offices. They also discourage from using the red flag on mailboxes, which act as "an alert there is mail waiting in your box," they added.