SACRAMENTO  (AP) — The California Department of Motor Vehicles says it is now in full compliance with federal requirements for so-called Real ID driver licenses or identification cards.

The state says it received notification of approval by the U.S. Department of Homeland Security on Wednesday.

Applicants initially had to provide one proof of residency, but Homeland Security later decided two are needed.

Real ID cardholders who only submitted one proof of residency will receive an address verification letter that must be signed and mailed back.

Letters will be sent to approximately 3.6 million holders of the 4.1 million Real ID cards issued to date.

Americans will need to show a valid passport or federally approved document such as a Real ID to board airline flights in the U.S. starting on Oct. 1, 2020.