(CNN) — Two hostages of suspected Somali pirates died after a rescue effort by the Danish navy, an operation that freed 16 other hostages, the Danish military said Tuesday.

A Danish warship, the Absalon, tasked with patrolling waters near the Somali coast to ward off piracy, fired warning shots against what the navy described as a pirate “mother ship” that failed to stop, the military said.

Military personnel then boarded the vessel and found 17 suspected pirates and 18 hostages, two of whom were critically wounded.

Despite medical treatment, navy doctors were unable save their lives, the military said. An investigation into the deaths is under way.

The hostages had been the original crew of what became the mother ship.