Italian authorities are puzzled over the bodies of 26 Nigerian teenage girls found dead in the Mediterranean Sea on Sunday, according to the Washington Post.

The International Organization for Migration reports that 2,715 migrants have died or gone missing trying to reach Italy by boat this year, but the fact that all the bodies recovered are female has raised some red flags.

Though "the cause of death appears to be by drowning," according to NPR, Italian authorities told CNN that they are now investigating the possibility that the girls had been sexually abused and tortured.

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“Most Nigerian women who arrive in Italy are already victims of trafficking, many have been subjected to serious sexual exploitation on their journey. Many are forced into prostitution in Libya,” said Simona Moscarelli, an expert with the IOM, told the Guardian last year.

The victims, all between the ages of 14 and 18, were found floating near the remains of a rubber dinghy, the head of Salerno’s police force, Lorena Ciccotti, told CNN. Some survivors were found hanging onto boat, among the bodies.

The BBC reports that 23 of the young women had been on a boat with 64 other people who were rescued alive and were found during one rescue operation. The bodies of another three women were later found on a separate mission, according to NPR. A police official told NPR that the women may have been thrown off the boat and left to drown.

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Officials believe the girls, who were originally from Nigeria, had attempted to reach Europe from Libya, the Washington Post reported. 

Police officials told the Agence France Presse that human traffickers do not usually use this route to smuggle women into Europe because "Loading women onto a boat is too risky, the traffickers would not do it as they could lose all their 'goods' — as they describe them — in one fell swoop," Salvatore Malfi, Salerno’s police prefect, told the AFP. 

Authorities are, nevertheless, investigating the possibility that the women were being trafficked.

The IOM’s data paints a slightly different story, showing that the trafficking of Nigerian women by sea may be more common than the Italian police believe.

The IOM, part of the United Nations, said last year that the trafficking of Nigerian women by boat from Libya to Italy had evolved into an unprecedented “crisis,” with 80% of Nigerian women reaching Italy being sex trafficked across Europe.

Read more: Refugee and Migrant Children Are Being Trafficked at Record Rates, UN Says

The sea route is often used by refugees, but only 42% of the refugees who have tried to cross the Mediterranean to reach safety this year have survived.

The girls’ bodies were recovered by a Spanish navy ship as part of the European Union’s “Navfor Med” initiative, which also rescued 402 migrants who survived the dangerous sea journey, including one Nigerian girl who gave birth onboard the ship shortly after being rescued. The rescue operation was one of four conducted this past weekend, the IOM reports.

So far, nearly 114,000 migrants have arrived in Italy having crossed the Mediterranean, the IOM says. And more people will likely attempt the perilous journey as they flee continued conflict and poverty, making their way through Libya, which has become a major center for human trafficking, CNN reports.

In the meantime, authorities are awaiting autopsies reports, which they hope will help reveal the girls’ fates.