EFFORTS to establish the exact number of people still missing from the Costa Concordia could be complicated by unregistered passengers who may have been on board the stricken vessel, the head of the search operation has admitted.
Around 20 people are still missing, including French passengers, an elderly American couple, an Italian father and his five year old daughter and Indian and Peruvian crew members.
As the death toll from the disaster rose to 13 after divers recovered a woman's body from the stern of the luxury liner, which rammed into rocks off the island of Giglio, Franco Gabrielli, head of the Civil Protection Authority, said there could have been "clandestine" passengers on board.
"There could have been X persons who we don't know about who were inside, who were clandestine" passengers aboard the ship, said Mr Gabrielli.
The family of a Hungarian woman who was not registered on the ship, insist that their daughter telephoned them from the Concordia and that they have not heard from her since, Mr Gabrielli said.
The suggestion of clandestine passengers raises the possibility that either a crew member or a passenger smuggled a friend or companion on board the ship, which rammed into rocks off the island of Giglio 10 days ago.
"Passengers might have been invited on board by a crew member at the last minute," said Francesca Maffini, Mr Gabrielli's spokesman.
Sources said that a second possibility was that someone could have turned up at the last minute and been registered in the correct manner, but the information might not have been relayed to Costa Cruises in Genoa by the time the ship smashed into rocks just two hours after its departure. That possibility was denied by Genoa-based Costa Cruises.
Davide Barbano, a spokesman, said all passengers would have been registered and photographed on boarding the vessel when it left the port of Civitavecchia, north of Rome, at the start of what was meant to be a week-long Mediterranean cruise.
Another possibility is that some crew members were not registered properly because they were working on the ship illegally. Many of the crew were from India, Bangladesh, Indonesia and the Philippines.
Shipping experts said that was unlikely however, because security checks had been significantly tightened in the wake of anti-terrorism legislation introduced in 2003, known as the International Ship and Port Facility Security Code.
"It is extremely unusual in this day and age in the Mediterranean to have an unregistered worker. If it is proved to be the case it will raise very, very serious questions about management practice. That would put the shipping industry in a very bad light," said Capt Syamantak Bhattacharya, a maritime academic based at the University of Plymouth.
The suggestion that people could have been smuggled on board mystified Phil Davies, a cruise expert.
"Manifests are put together well in advance of the voyage, detailing the number of passengers and crew.
"It would by highly unusual for someone to be on board without the company knowing who they were.
"Passengers have a card which doubles up as a key for the cabin and also acts as security for getting on and off the ship. You need to have the card with you.
"Also as soon as you get on board they will take your photo. There are two gangways, you cannot just sneak somebody on board. The other thing is that cruise ships are a cashless society, everything is charged to the card. The whole thing is a mystery to me."
The bodies which have not yet been identified could eventually tally with people on the missing list.
Of the 13 confirmed dead, eight have been identified, but the remaining five are badly decomposed because they were in the sea for several days. Mr Gabrielli said that one of the bodies could be that of the Hungarian woman.
Both Costa Cruises and its parent company, US-based Carnival, are members of the Cruise Lines International Association, the industry body which sets safety and security rules.
This means there should be "100 per cent screening" of passengers, crew and luggage. Security checkpoints are also required for all passengers on embarking and disembarking.
Each cruise ship has a dedicated security officer and staff whose sole function is the security of its passenger and crew."
Mr Gabrielli said he had requested medical opinion on how long passengers or crew might be able to survive in the freezing cold conditions on board the 17-deck ship, which is half-submerged in shallow water off Giglio's coast, close to its tiny harbour.
He was given overall command on Friday of the multiple emergency services involved in the search and salvage operation, including civilian salvage experts from the Netherlands, police and Carabinieri, the Italian navy and the coast guard.
He has asked experts to determine whether it might be possible to carry out the search operation and the fuel removal at the same time.