Firefighters have boarded the charred Norman Atlantic ferry in the Italian port of Brindisi to put out residual fires and allow a search for more possible victims.
A blaze broke out on the ferry on Sunday as it was travelling from Greece to Italy, killing at least 11 people.
Italy says 477 people were rescued, most by helicopters that plucked survivors off the top deck in gale-force winds and carried them to nearby boats.
The probe into the disaster widened yesterday.
In addition to the ship's captain and the head of the company that built the ferry - both Italians - the prosecutor's office in Bari put two other crew members and two representatives of the Greek ferry line Anek, which rented the Norman Atlantic, under investigation, the Italian news agency Ansa reported.
Towing the ferry overnight across the choppy seas of the Adriatic took 17 hours.
Once the blaze is fully extinguished and the boat stabilised in port, firefighters will start searching for bodies.
A prosecutor must be on board for the start of the official inspection of the wreck to help determine the blaze's cause.
Prosecutors fear unregistered migrants were smuggled aboard in trucks and might have died in the flames and smoke.