Some employees of the US Citizenship and Immigration Service were unable to log on to their computers Tuesday after the agency experienced a network outage.

The outage was caused by an expired certificate, according to an official at the Department of Homeland Security, the CIS's parent department. Certificates are digital IDs that verify someone is allowed to access a computer network.

The outage was reported at 6:30 a.m. ET, the DHS official said, and all services were restored by 9:30 a.m.

Four buildings in the Washington, DC, area were affected.

The computer problems were earlier reported by Reuters.

First published Feb. 21, 1:01 p.m. PT.

Update, 3:45 p.m.: Adds confirmation from the DHS.