Five days after a fire on board the Zahro Express that claimed at least 23 lives, the authorities are still investigating what caused the blaze.

On Thursday, investigators from the National Police's forensics laboratory and the National Transportation Safety Commission (KNKT) were investigating the charred remains of the vessel at a dock in Muara Angke, North Jakarta.

A KNKT officer, who spoke on the condition of anonymity, told The Jakarta Post that the KNKT was examining the vessel’s engine room.

"The source of the fire was the boat's engine room," he said without giving further details.

(Read also: Passenger overload suspected to be contributing factor in Zahro disaster)

Several officers were seen taking items from the engine room for further investigation.

The Zahro Express caught fire on Sunday morning when traveling from Muara Angke Port in North Jakarta to Tidung Island in Thousand Islands regency. Most of the passengers intended to celebrate New Year's Eve on Tidung Island. Of the passengers on board, 130 survived, 23 died and 17 were declared missing.

On Wednesday, authorities found the body of the one of the 17 missing passengers.

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