CATANIA, Sicily — Prosecutors began building the case against the traffickers responsible for what might be the Mediterranean's worst migrant disaster as European Union leaders mulled a new plan to stem the tide and undermine smugglers' finances and most precious assets: the boats.

Overnight Tuesday, prosecutors boarded the rescue ship that brought the 27 survivors of the weekend disaster to Sicily and arrested the Tunisian captain and a Syrian crew member of the smuggler ship that capsized.

They are accused of favoring illegal immigration and the captain was also accused of reckless homicide in the deaths of between 700 and 900 migrants.

The survivors were brought to a migrant holding center in Catania and were "very tired, very shocked, silent," according to Flavio Di Giacomo of the International Organization of Migration.

"They told us that they were 800 on board," he said. "Everyone told us that there were about 800 so we have to say that 800 people have died at this point."

The coast guard, meanwhile, reported that it saved some 638 migrants in six different rescue operations on Monday alone. Other rescue operations were taking place Tuesday, including one south of Calabria in which two merchant ships were asked to lend a hand.

The weekend deaths have jolted the European Union into taking action, with Italy demanding that it not be left alone to shoulder the burden of rescues and that the EU focus on preventing the boats from leaving Libya in the first place.

Ahead of an emergency EU summit Thursday, EU foreign and interior ministers approved a 10-point plan at a meeting in Luxembourg that calls for the beefing-up and expansion of the current EU border patrol mission, and embarking on a "systematic effort to capture and destroy" smuggler boats.

It calls for closer law-enforcement coordination to trace smugglers' funding, which prosecutors have said often evades traditional bank transfers in favor of informal Arab hawala networks, in which migrants' relatives in Europe pay local brokers for each leg of the journey.

"We are facing an organized criminal activity that is making lots of money, but above all ruining many lives," Italian Premier Matteo Renzi said at a joint news conference Monday with Malta's prime minister, Joseph Muscat. He compared their activity to that of slave traders of centuries past, "unscrupulous men who traded human lives."

Italy had launched a robust and expensive search-and-rescue mission in 2013 after some 366 migrants drowned off the island of Lampedusa. The politically unpopular Mare Nostrum operation ended last year, and the EU's Frontex border patrol mission took charge. But its limited mandate and resources have prevented it from being effective in saving lives.

The EU plan also calls for member states to ensure all migrants are fingerprinted. As it is, many migrants merely pass through Italy without being fingerprinted or applying for asylum, preferring instead to pay smugglers to get them to northern Europe where they apply for asylum and have better job opportunities.