SACRAMENTO (AP) — Production problems with the manufacture of California’s new high-tech driver’s licenses have led to lengthy delays for tens of thousands of drivers waiting for new cards.

The Sacramento Bee reports that internal emails between the Department of Motor Vehicles and its Massachusetts-based contractor, L-1 Identity Solutions, indicate that the licenses’ enhanced security features have contributed to the slowdown.

Those features include hidden photos visible only by ultraviolet light and laser-engraved signatures. The state says the licenses are the most advanced identity cards in use anywhere.

The delays started immediately after the DMV began issuing the new cards in October, and have stretched up to four months in some cases.

DMV officials tell the Bee they are slowly but steadily reducing the backlog, which briefly swelled to 850,000 earlier this month.

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