Story highlights Energy minister says 90% of Istanbul's power has been restored

Turkish Prime Minister: All possible causes of outage under investigation, including terrorism

(CNN) A massive power outage spread across Turkey on Tuesday, blacking out a broad swath of the country and affecting some flights.

Prime Minister Ahmet Davutoglu said all possible causes were under investigation, including terrorism.

Trams and subways were idled in Istanbul -- with more than 14 million inhabitants, the largest city in the country and the fifth largest in the world. Homes and offices were left without electricity.

"I am sitting in my apartment," CNN producer Gul Tuysuz reported from Istanbul. "No lights. No electricity."

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