An expired security certificate was to blame for some Homeland Security Department employees being locked out of their computers Tuesday morning, as first reported by Reuters.

In a statement, a DHS official told Reuters an “expired DHS certificate” caused a temporary network outage in four U.S. Citizenship and Immigration Services facilities in the Washington, D.C., area.

For approximately three hours, personal identity verification cards feds and contractors use to access federal information systems didn’t work in those facilities before service was restored around 10 a.m.

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According to a DHS official, domain controller credentials expired on Presidents Day—a federal holiday—and steps to ensure network redundancy weren’t taken. That means the network issues users experienced were preventable.

“We are working to track all device certificate issuance and expirations to ensure future lapses of service do not occur again,” the official told Reuters.

DHS did not respond to Nextgov regarding how many users were affected by Tuesday’s glitch.