Up to 100 children may have died in the weekend’s catastrophic shipwreck in the Mediterranean, a relief agency has said as prosecutors in Sicily arrested the alleged commander of the wooden fishing vessel and a member of his crew.

Survivors of the wreck said that 60 of the children on board the boat were not accompanied by adults, a spokeswoman for Save the Children said. Four of the survivors were teenagers, aged 16 and 17, travelling on their own and without any family in Europe, she added.

“They said that what happened with the shipwreck was terrible but that they are the lucky ones. They said they were just very happy to be here,” the spokeswoman said.

New details emerged on Tuesday about the nature of the wreck, those allegedly in charge of the vessel and the number of fatalities, which could total between 400 and 950, making it one of the worst such maritime disaster. Italian authorities said the high number of deaths could be due to the fact that many of the migrants were locked in the hold of the boat when it capsized.

Mohammed Ali Malek, a 27-year-old Tunisian national who was the alleged captain of the boat, and Mahmud Bikhit, a 25-year-old Syrian, were charged with multiple manslaughter. They were being held in detention in the Sicilian port city of Catania, according to Italian police.

Prosecutors said in a statement they would be questioning the defendants on Tuesday evening. Both would be defended by the Catania lawyer Massimo Ferrante. He was not immediately available for comment.

They were charged by police after the Gregoretti, the Italian coast guard vessel carrying 27 of the wreck’s survivors, arrived in Catania late on Monday night. Another survivor had already been airlifted to a hospital in Sicily.

News agency photographs taken in Malta, where the Gregoretti docked on Monday morning before continuing on to Italy, showed Malek sitting on the deck and watching as white body bags were removed by health officials.

Investigators were going through survivors’ testimony to build their case against the two suspects and have not ruled out trying to recover the sunken vessel.

Prosecutors said they had pieced together some basic facts. The fishing boat is believed to have left Tripoli on the evening of 16 April. Some of the migrants had been held for as long as 30 days before the journey began. The smugglers are believed to have kept the waiting migrants on a farm near the departure point before transporting them to the vessel in vans.

In one case, one of the migrants was allegedly hit by a stick because he had distanced himself from the crowd in order to urinate, but had not received permission from the smugglers to do so.

The cost of transport, according to the prosecutors, was 500-1000 Libyan dinars (£250-500) per person.

Police said there were two likely causes for the wreck. First, that the captain carelessly steered the ship into an oncoming Portuguese cargo ship, the King Jacob, which was coming to the vessel’s aid. The captain of the King Jacob, which has helped in other migrant rescue operations, has repeatedly denied there was a collision.

Second, the ship may have capsized when migrants on the overcrowded boat moved to one side.

Families of many of the still-anonymous victims have started to approach aid groups such as the Red Cross, which said on Tuesday it had received 20 calls and six visits from people who feared their relatives were among the dead.

The calls were from the countries from where the victims are believed to have originated: Somalia, Eritrea and Syria, among others. Some migrants, who are already living in Sicily, went in person to the Red Cross centre to ask about relatives.

“These people were very distressed. You can imagine – missing a family member and fearing them dead,” said Silvia Dizzia, a Red Cross volunteer in Catania involved in the Restoring Family Links programme. “What a tragedy. You could hear the desperation in their voices.”

After providing information about the people they were looking for, family members offered the Red Cross documentation to try to prove they were related to possible victims, but the agency said that, given how many migrant boats kept landing in Sicily, it was still too difficult to discern the fate of any individual.

“It’s hard to know whether those people may have been on another boat and alive somewhere. It is a very difficult situation,” Dizzia said.

Survivors of the disaster were at a detention centre in the town of Mineo. The site used to be a residential base for US Navy personnel, but now houses migrants.

The International Organisation for Migration, another aid group, said it believed about 1,727 migrants had died so far this year in their attempts to reach Europe from Africa. That toll is about 30 times higher than last year’s total at this time, it said.

The spokesman Joel Millman told journalists in Geneva: “IOM now fears the 2014 total of 3,279 migrant [deaths] on the Mediterranean may be surpassed this year in a matter of weeks, and could well top 30,000 by the end of the year, based on the current death toll. It could actually be even higher.”

The IOM said there was no more information available about two other vessels believed to have been in distress on Monday carrying a total of about 450 migrants.

Families seeking to identify or find information about loved-ones can email Italian authorities at this email address: naufragio.wreck.procura.catania@giustizia.it