ISTANBUL (AP) — Turkish authorities say at least 20 people have died and 11 others are in critical condition after a tour bus tipped over.

The governor of Mugla province, Amir Cicek, said the bus tipped over at a road bend as it was traveling from the coastal province of Izmir to the Marmaris tourist destination.

Cicek said the passengers were all Turkish.

The serpentine road, on the side of a cliff, has been closed off to traffic, causing massive congestion.

The governor says the cause of the accident is unclear, and will be under investigation.