(Des Moines) — MidAmerican Energy officials say they are investigating the cause of a power outage affecting thousands of area residents Wednesday morning. Spokesman Geoff Greenwood, said customers in portions of six counties lost electricity at around 9 a.m.

Persons served by substations in the Red Oak, Griswold and Emerson areas, or about 6,000 customers, were affected. Power was restored in two stages, with everyone back online by around 10:00-a.m.

Greenwood said the cause of the outage had not immediately been determined.