The United Nations refugee agency said Tuesday that more than 800 people were believed to have drowned in the weekend sinking of a boat packed with migrants trying to reach Europe, making it the deadliest such disaster in the Mediterranean.

New details of the tragedy emerged as the U.N. High Commissioner for Refugees and other aid agencies interviewed some of the 28 survivors who arrived overnight in Catania, Sicily.

Survivors put the number of passengers on board the three-deck fishing trawler at 850, according to UNHCR spokesman Adrian Edwards in Geneva. Only 24 bodies have been recovered.

"From available information and the various accounts we've had, UNHCR now believes the number of fatalities to have been over 800, making this the deadliest incident in the Mediterranean that we have recorded," Edwards told reporters in Geneva.

Prosecutors in Catania said a special email address had been set up for relatives, friends and acquaintances of the victims in hopes of determining who was on board.

Still, they acknowledged the exact death toll may never be known. `'No facts have emerged that can be helpful to determine in a more precise way the number of dead," prosecutors said in a statement.

The International Organization for Migration said the rate of migrant deaths in the Mediterranean this year is far higher than in 2014, when a total of 3,279 migrants died. That, in turn, was much higher than in 2013, when around 700 people died, the agency said.

So far this year, 1,776 have died, according to the U.N. refugee agency, which estimates that 219,000 people made the crossing in 2014.

The 2015 death toll "could well top 30,000," said Joel Millman, the IOM spokesman. "We just want to make sure people understand how much more ... rapid these deaths have been coming this year than last year."

Among the arrivals overnight were two suspected smugglers, who were immediately detained for investigation of aiding and abetting illegal immigration. The captain also was being investigated for multiple counts of manslaughter and causing a shipwreck. The two were to be questioned later Tuesday.

Survivors told aid workers the wreck was caused when one of the smugglers crashed the boat against the Portuguese-flagged King Jacob container ship that had responded to a distress call, according to UNHCR spokeswoman Carlotta Sami.

"The survivors said that the person who was steering the boat, their smuggler, was navigating badly, and he did a bad move that made it crash against the bigger ship," Sami said by telephone from Sicily.

Prosecutors said that after the trawler's captain, a 27-year-old Tunisian identified as Mohammed Ali Malek, rammed the Portuguese vessel, terrified migrants rushed to one side of the overcrowded boat, which was already unbalanced from the collision. The trawler pitched in the water before finally tipping over, and sank.

Most on board were unable to escape because they were locked below deck on the trawler's lower two levels. Hundreds more were squeezed on the upper deck, according to prosecutors.

Catania prosecutor Giovanni Salvi on Tuesday backtracked on an earlier statement that the container ship's inexperience had contributed to the tragedy, stressing that crew members had done their job.

Sami praised the Portuguese ship, which had previous experience in a handful of other migrant rescues.

Mistreatment began before the migrants even set foot aboard the doomed boat on the night of April 16, prosecutors said in a statement, citing survivor accounts.

The migrants had been held for as long as 30 days on a farm near where the boat was docked before being transported in groups of about 30 in trucks to the embarking point, it said.

`'In one instance, one of the migrants was allegedly struck with a club because he stepped away" to go to the bathroom, the statement said.

The weekend deaths have jolted the European Union into trying to come up with a plan to address the crisis, 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.

Combatting the smugglers by arresting the ringleaders and destroying their boats is emerging as a key part of Europe's 10-point proposal for an emergency summit in Brussels on Thursday. Italy has arrested more than 1,000 smugglers, most of them the boats' navigators and not the masterminds.

Survivors of the weekend disaster were brought Tuesday to a migrant holding center in Catania and were "very tired, very shocked, silent," according to Flavio Di Giacomo of the IOM.

Sami said all the survivors were men, several of them adolescents. `'They are very confused, fragile and scared," she said.

The coast guard, meanwhile, reported that it saved some 638 migrants in six different rescue operations on Monday alone. On Tuesday, a further 446 people were rescued from a leaking migrant ship about 80 miles (130 kilometers) south of the Calabrian coast.

The smugglers use a variety of boats for their crossings, from rubber Zodiac-type boats to wooden fishing vessels and even old cargo ships. They are almost always overcrowded to maximize the revenue of the smugglers, who charge between 1,000 and 1,500 euros (dollars) for the crossing from Libya, where most trafficking operations originate.

Not all those turning to smugglers to escape conflict or violence are risking their lives in unseaworthy boats. Police in Ragusa, a Sicilian port town, said they arrested three Syrians connected to a Turkish-flagged luxury yacht that charged passengers $8,500 for the trip from Turkey to Sicily. Among the Syrian and Palestinian passengers were 23 children.

Selfies and other photos snapped by passengers helped police identify the smugglers, police said in a statement. They estimated that the organizers were paid some $800,000 in total for the trip. Authorities discovered the yacht was a smuggling boat when two merchant ships were called out to aid a boat in distress.

The EU plan calls for closer law-enforcement coordination to trace smugglers' revenue sources, which prosecutors have said often evade traditional bank transfers in favor of informal networks, in which migrants' relatives in Europe pay local brokers for each leg of the journey.

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.