Brindisi - Wearing gas masks against the smoke, Italian firefighters and investigators boarded the charred Norman Atlantic ferry on Friday and retrieved a data recorder they hope will help them discover what caused a deadly blaze.But with some parts of the ferry still burning, they emerged hours later to admit they had to put off for at least a day the search for any more bodies in the maritime disaster that has already killed 11 people. The team will attempt to go back on board on Saturday.Greece says 19 people are still unaccounted for after a fire broke out on Sunday as the ferry travelled from Greece to Italy, and disputes Italian claims of a higher number of missing. Italy says 477 passengers and crew were rescued from the burning ferry, most by helicopters operating in gale-force winds.Both nations fear the ferry car deck where the fire started could contain more bodies, possibly those of unregistered migrants trying to slip into Italy.'Impossible to get inside'The badly damaged ferry was towed for 17 hours across the choppy Adriatic Sea before docking on Friday at the southern Italian port of Brindisi. A second tug was tied in with the ship to stabilize the wreck. One side of the ferry was blackened by smoke and an acrid smell was noticeable dockside.Investigators began work on Friday by taking photos and video of the ferry's smoky interior. After several hours, prosecutor Ettore Cardinali stepped back ashore, took off the dust-filtering mask covering his nose and mouth, and told reporters the team couldn't get into the crucial car deck."For the time being, it is unfortunately impossible to get inside ... for safety reasons, we cannot verity firsthand what's inside," he said.But he did say investigators had retrieved the black box recorder and promised to extract data from it.Captain under investigationFirefighters say they will not start searching for bodies until the blaze is fully extinguished - and could not give an estimate of when that would be."There are cars and trucks and other things that are still slowly burning, which ... could still go ahead for a long time," Brindisi Fire Commander Michele Angiuli told reporters.Four more people, meanwhile, were put under investigation on Friday 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 on Wednesday, said Capt. Argilio Giacomazzi told prosecutors that crews didn't properly follow his orders in lowering the lifeboats and that the car deck had too many vehicles.Bari prosecutors have declined to say what the captain said, citing laws governing investigations.Italian TV said passengers noted that 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.Fears about migrants hidden on the huge ferry are based on reality. In 2014, Italy says it rescued or discovered some 170 000 migrants and asylum seekers at sea as they tried to slip into Europe.