Italian firefighters and investigators have boarded the charred Norman Atlantic ferry as they try to find out what caused the deadly blaze and search for more possible bodies.

The badly damaged ferry was towed across the choppy Adriatic Sea for 17 hours before it docked at the southern Italian port of Brindisi.

Despite the fact that a slow combustion fire was still burning on the ferry, officials deemed it safe enough for investigators to go aboard wearing gas masks against the smoke.

Brindisi Fire Commander Michele Angiuli told reporters it was too soon to know if investigators would find more bodies.

"What we know is that there are cars and trucks and other things that are still slowly burning, which ... could still go ahead for a long time," he said.

The fire that broke out on Sunday as the ferry travelled from Greece to Italy killed at least 11 people. Italy says 477 passengers and crew were rescued, most by helicopters that plucked them off in gale-force winds and carried them to nearby boats.

Greece says 19 people are still unaccounted for and disputes Italian claims of a much higher number of missing. Both fear the car deck where the fire started could contain more bodies, possibly those of unregistered migrants trying to slip into Italy.

Investigators began their work by taking photos and video of the smoky interior. Brindisi port Captain Mario Valente said they would be looking for data recorders as well.

One side of the ferry was blackened by smoke and an acrid smell was noticeable dockside. The firefighters will not start searching for bodies until the blaze is fully extinguished.

Meanwhile, four more people were put under investigation by the prosecutor's office in Bari. In addition to the ship's captain and the head of the company that built the ferry - both Italians - two other crew members and two representatives of the Greek ferry line Anek, which rented the Norman Atlantic, are under investigation, the Italian news agency Ansa reported.

Italian newspapers, reportedly quoting from transcripts of the captain's questioning, 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 say only three crew members should go with the evacuated passengers.

Italy says it rescued or discovered 170,000 migrants and asylum seekers at sea last year as they tried to slip into Europe.