The loss of the girls – believed to be Nigerians aged 14 to 18 – has been described by the Nigerian foreign ministry as "a monumental loss and a sad moment for our country".
According to the post-mortems carried out in Salerno in southern Italy, 25 of the girls died of asphyxiation in the water, most of them when the inflatable dinghy they were travelling on sank.
One girl suffered a wound to her liver. Two of the dead were pregnant.
The autopsies found no recent trace of physical or sexual violence.
According to the UN, 80 percent of young Nigerian women arriving in Italy are already in the clutches of prostitution networks, or quickly fall under their control.
The bodies of the victims were found floating in the water by a Spanish military ship and brought to Italy on November 3rd after two separate rescue operations.
In both cases, dozens of other migrants – mostly men but also women – were also rescued after they tried to cross to Europe from Libya.
At least 100 people are still missing at sea, according to investigators in Palermo. They are feared to have drowned in the same shipwreck that killed the 26 girls.
Family members who survived the disaster have identified some of the victims. In other cases, investigators managed to contact relatives after discovering phone numbers inside the victims' clothing.
Funerals for the 26 girls will take place on Friday in Salerno, where a day of mourning has been declared.
A white rose will be placed on each coffin, while smaller roses will be placed for the two unborn children.
Nigeria has called for an international investigation into the incident.
According to Italy's interior ministry, more than 111,700 people have reached the country by sea in the first 10 months of 2017, a decrease of 30 percent from the same period last year.