Italian navy divers blew open access points in the wreck of the Costa Concordia on Monday as rescuers debated whether to call off their search for at least 19 people still missing.
Italian media meanwhile reported that the luxury liner's captain, Francesco Schettino, who faces charges of multiple manslaughter, had tested negative for drugs, while his lawyer said there could be other suspects in the tragedy.
"The investigations are in full flow also to determine possible other responsibilities of third parties who could at least have had a role" in causing the shipwreck, Bruno Leporatti was quoted as saying.
Schettino has claimed that the risky route he took close to the Tuscan island of Giglio in a showboating manoeuvre was agreed beforehand with his superiors at Costa Crociere -- a subsidiary of US-based giant Carnival Corp.
He has also said that he kept the company's crisis officer fully informed about the scale of the disaster within minutes of hitting rocks, while the order to abandon ship was only given more than an hour later.
The shipping line declined to comment when contacted by AFP.
Navy divers set off small quantities of explosives at depths of up to 18 metres (59 feet) to allow access to decks four and five of the half-submerged luxury 114,500-ton cruise ship, which crashed 10 days ago.
"A team of divers is searching inside the vessel now," said Alessandro Busonero, a spokesman for the navy, explaining that the series of small blasts had smashed through window panes of three-centimetre (one-inch) thick glass.
Busonero also said an oceanographic ship, the Galatea, had arrived on Giglio to help search for objects or bodies on the sea bead with the help of high-resolution imaging equipment and calculations of currents.
Thirteen people have been confirmed dead in the tragedy so far, including five bodies recovered that have not yet been identified. Officials have said there may also have been a Hungarian woman and others aboard as stowaways.
Some experts believe the operation to pump out 2,380 tonnes of heavy fuel oil from the ship's tanks cannot start until search operations are suspended because it could destabilise the vessel and cause it to sink entirely.
"Today will be a decisive day. We're holding our breath to see whether it will be possible to start pumping out the oil while continuing the search for missing people," the mayor of Giglio, Sergio Ortelli, told AFP.
"Obviously everybody would rather carry on the search, but it's hard to believe that by some miracle there is someone still alive now," he said.
Dutch firm Smit, one of the world's largest marine salvage companies, has been ready to pump the fuel out of the wreck for a week, but a representative on the island said it was "unlikely" it would begin Monday.
Environmentalists have warned of a potential ecological disaster if there is a leak of fuel from the ship -- which was at the beginning of a seven-day cruise when it hit rocks close to the shoreline and half-capsized.
The environment ministry has said toxic products from the ship -- including detergents and solvents -- have already started seeping into the sea.
The island, part of Europe's biggest marine sanctuary, is a popular holiday spot with pristine sandy beaches and spectacular rocky shores.
The Costa Concordia had 4,229 people on board from more than 60 countries.
Confusing reports also emerged over Schettino's actions in the hours after the crash as the owner of a hotel on the island said he saw him hand over what looked like a personal computer to an unknown blonde woman.
Paolo Fanciulli, 45, who owns the Hotel Bahamas, said the woman swept into the lobby, took the bag and ushered Schettino away from journalists at around 11:30 am (1030 GMT) on Saturday, the morning after the shipwreck.
Newspapers meanwhile reported that Schettino was on the bridge in a jovial mood with various people who should not have been there when the impact happened and that the captain went into his cabin briefly after the crash.