The bodies of an American couple, two German nationals and an Italian crewman who died when the Costa Concordia capsized on January 13 off the Italian coast have been identified by Italian authorities.

The names of Barbara and Gerald Heil of Minnesota and Christina Mathi and Norbert Josef Ganz from Germany were released by the Grosseto prefecture after weeks of DNA testing and forensic investigations. Giuseppe Girolamo was named as the dead crewman.

At least 30 people died when the cruise ship hit rocks and turned on its side off the Italian island of Giglio. A female Italian passenger and an Indian crewman are still missing.

Costa Concordia's captain Francesco Schettino is still under house arrest as the investigation continues. He may face criminal charges for alleged manslaughter, causing a shipwreck, abandoning ship, failing to report an accident and damaging the natural habitat of Giglio, which is a protected park.

Schettino's lawyer questioned Italy's highest court's ruling to keep the captain under house arrest, claiming the suspect had "full trust and respect in the judges and awaits the supreme court's decision with serenity".

Schettino's first officer, Ciro Ambrosio, and six other officers are under investigation for alleged manslaughter, shipwreck and failure to report the accident.

The salvage operation to move the capsized liner away from Giglio will begin in May, civil protection official Fabrizio Curcio said. Two companies are on the shortlist to carry the operations.

Meanwhile, the vice-chairman of Carnival, the company that owns the Costa Concordia, said that the Costa Cruise division remained a "powerful brand in Europe".

Carnival, which is the world's biggest cruise ship operator, saw pricing and booking volumes plunging in the aftermath of the tragedy. But Howard Frank said Costa Cruises has recovered to better levels in the past few weeks.

"As we get through this process we think it will come back quite nicely," he told the annual shareholder meeting in Florida. "We are very confident about Costa."