CATANIA, Sicily — Prosecutors said Tuesday that they arrested the captain and a crew member of the boat in which as many as 900 people are feared to have drowned in the unremitting waves of migrants seeking to escape from war-torn Libya.

Even as the search continued for victims of the weekend disaster, coast guard ships rushed to respond to new distress calls on the high seas — two off Libya and a third boat that ran aground near Greece.

Assistant Prosecutor Rocco Liguori said the Tunisian captain and Syrian crew member were arrested aboard the rescue boat that brought 27 survivors from the deadly shipwreck to Sicily. The two were charged with favoring illegal immigration and the captain was also charged with reckless multiple homicide in relation to the sinking.

Decrying what he called an “escalation in these death voyages,” Italian Premier Matteo Renzi urged Europe to put the focus on preventing more boats from leaving Libya, the source of 90 percent of migrant traffic to Italy.

“We are facing an organized criminal activity that is making lots of money, but above all ruining many lives,” Renzi said at a joint news conference 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.”

The European Union foreign affairs chief, Federica Mogherini, said this weekend’s appalling human toll — which, if verified, would be the deadliest migrant tragedy ever — had “finally” fully awakened the European Union to the evils of human trafficking.

The EU has been under increasing criticism for lagging in its response to the crisis, with two shipwrecks believed to have taken the lives of as many as 1,300 migrants in the past week. Some 400 people are believed to have drowned in another capsizing on April 13.

Stopping the traffickers will be a key item on the agenda when EU leaders meet in an emergency summit Thursday in Brussels, along with a proposal to double spending on sea patrols off Europe’s southern border. The 10-point plan includes a proposal to take “civil-military” action modeled on Europe’s anti-piracy operation off the coast of Somalia, to capture and destroy boats used by traffickers.

Meanwhile, new details emerged about the weekend disaster, with Italian prosecutors saying hundreds of migrants were locked below deck unable to escape when the rickety boat capsized off the coast of Libya.

Speaking at a news conference in Catania, Sicily, prosecutor Giovanni Salvi said “a few hundred were forced into the hold and they were locked in and prevented from coming out.” He said hundreds more were locked on a second level of the boat, which also had hundreds of migrants squeezed into its upper deck.

Salvi said the migrants rushed to one side of the boat as they saw a Portuguese-flagged container vessel approach, with the promise of rescue contributing to the disaster.

“Merchant ships don’t have adequate training for rescues in the seas,” Salvi warned. “The fact is, sea rescues are difficult and require professionalism.”