The boxes were forced open with pry tools and locks were cut with bolt cutters to get to delivered mail, according to officials.
Emergency officials said an inspector from the U.S. Postal Inspection Service has been assigned to investigate the rash of thefts and will be working with the San Mateo County Sheriff's Office.
Anyone with information about the mail thefts is asked to email oyochsmcgov.org.
Residents are asked to report any suspicious vehicles or people breaking into mailboxes and to share a full description of suspects to San Mateo County sheriff's dispatch at (650) 363-4911.
Mail theft is often associated with check, credit card and identify theft. Stealing U.S. mail is a federal offense punishable by a lengthy term in federal prison, according to officials.
