Two lighters sank into Bay of Bengal at Thengarchar near Kutubdia on Wednesday with 2500 metric tonnes of clinker.
The two ships were MV Titu 18 and MV Titu 19.
BIWTA deputy director Md Selim said that around 1: 30 pm of Wednesday the two ships owned by Shah Cement were capsized due to severe storm.
Each ship has 12 crews. All the arews of MV 18 were rescued.
Bangladesh Navy rescued 21 crews of the ship. Among them 11 were sent to Zohurul Haque Base of Chattogram by helicopters.
Others are still missing, said Md Selim.
Shah Cement Shipping Agent Limited operation in charge Jahid Hossain said that a mother vessel was waiting in the sea with the clinkers to unload.
The two lighter ships went there and unloaded clinkers and when they were going back to Narayanganj and Munshiganj there was severe storm in the sea.
The two ships turned and sank into the sea.
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