The Tunisian captain of a boat that capsized off Libya on Sunday, killing hundreds of migrants, faces charges of reckless multiple homicide, Italian officials say.
He has also been accused, along with a Syrian member of the crew, of favouring illegal immigration.
The two were among 27 survivors who arrived in Sicily late on Monday.
The authorities say the disaster was caused by mistakes made by the captain and the ship being overcrowded.
Prosecutors in the Sicilian port of Catania said the boat had collided with a Portuguese container ship just before it capsized, but absolved the merchant vessel's crew of any responsibility.
They said the boat had keeled over after the collision which had been caused by steering mistakes by the captain and the panicked movements of the migrants on the 20-metre (66ft) former fishing trawler.
Carlotta Sami of the United Nations High Commissioner for Refugees in Italy was in Catania to meet the survivors. Some 800 people are thought to have died in the disaster, she said.
There were nationals of Syria, Eritrea, Somalia, Mali, Sierra Leone and Senegal on board, kept in three different layers in the boat.
"They left on Saturday morning around eight o'clock in the morning from Tripoli, and they started to have problems, and they were approached by merchant vessels during the night around 10 o'clock.
At some point, "the little boat lost its balance, and people started to move around. Those that were down wanted to come up and vice-versa, and many people fell into the water, and then the boat capsized," she said.
The two men questioned in connection with the disaster have been named as ship commander Mohammed Ali Malek, 27, a Tunisian, and crew member Mahmud Bikhit, 25, a Syrian.
The development comes after the EU set out a package of measures to try to ease the migrant crisis in the Mediterranean.
Search-and-rescue operations will be stepped up, and there will be a campaign to destroy traffickers' boats.
A homicide investigation has been opened into the disaster.
Separately, two of those rescued from a vessel carrying dozens of migrants that ran aground off the Greek island of Rhodes on Monday will be taken to the prosecutor's office, the BBC has learnt.
It is thought the two men, both Syrians, were in charge of the boat; they will face charges linked to illegally transporting 90 people to Greece, and responsibility for the deaths of three passengers.
At the scene: James Reynolds, Rome correspondent
The survivors stood still on the rescue boat. They looked exhausted. One shook hands with the mayor of Catania and put his hand to his chest in a gesture of thanks.
Francesco Rocca runs the Italian Red Cross: "They are under shock, completely shocked. They repeat their phrases about the fact that they are the only survivors on the tragedy.
"Some of them want to speak, some of them want to stay silent. You can imagine they are under a lot of pressure. It's the first time I see such a high level of shock. It's clear from their eyes."
Two survivors told rescue workers that they had managed to stay afloat by clinging to the bodies of their fellow passengers. Others said that the children on board drowned because they were trapped on the boat's lower two levels.
'Sense of solidarity'
EU foreign policy chief Federica Mogherini said the 10-point package set out at talks in Luxembourg was a "strong reaction from the EU to the tragedies" and "shows a new sense of urgency and political will".
"We are developing a truly European sense of solidarity in fighting human trafficking - finally so."
The measures include an increase in the financial resources of Frontex, which runs the EU's Mediterranean rescue service Triton, and an extension of Triton's operational area.
The EU had been criticised over the scope of Triton, which replaced the larger Italian operation Mare Nostrum at the end of last year.
Italian Red Cross chief Francesco Rocca told the BBC he hoped the international community would be able to find concrete solutions in the countries where migrants flee from.
"Most of them don't want to escape, they are forced to escape, they are escaping war, they are escaping [intense] hunger, so this is something that we cannot avoid.
"If we block one route, they will find another route, so this is something we have to face... not with only words or actions that don't match the concrete needs of the people."
Italian coastguard update on search-and-rescue operations
- Six hundred and thirty eight migrants brought to safety on Monday in six different search-and-rescue operations co-ordinated by the Italian coastguard
- Six different dinghies full of migrants all in Libyan waters
- Ninety-three of them, including 12 women and two children, were brought to Lampedusa by a customs ship
- At dawn on Tuesday, there was a distress call from a fishing boat full of migrants 80 miles (130km) south-east of the Calabrian coast on the Italian mainland. Two merchant ships provided assistance until two coastguard ships arrived. The coastguard gave the migrants life jackets and "calmed them", and they are being brought aboard an Italian navy ship.
Correction 29 April 2015: This report has been amended to clarify that the captain and crew member were being investigated but had not been formally charged.