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 today. 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.
Several hours after the Norman Atlantic arrived in Brindisi port, a second tug was tied to the ferry to help stabilise it.
Two firefighters went aboard to check for residual fires since smoke was still coming out. One side of the ferry was blackened by smoke, and an acrid smell was noticeable dockside.
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.
Italian newspapers, reportedly quoting from transcripts of the ferry captain's questioning on Wednesday by prosecutors, said Captain Argilio Giacomazzi told prosecutors that crews did not properly follow his orders in lowering the lifeboats and that the car deck had too many vehicles.
Italian TV reports said passengers claimed five crewmen were in the only lifeboat launched, in apparent violation of rules that specify only three crew members should go aboard with the evacuated passengers.