Updated

As they closed down the last island penal colony in the hemisphere after more than a century, Mexican authorities faced a conundrum: they had lost track of 16 inmates registered at the Islas Marias prison.

Most were later located at other prisons on the mainland, Francisco Garduno, the head of Mexico's prison system, said Friday. They had apparently been transferred from the penal colony but not struck from its rolls.

But Garduno said two inmates were never found and are presumed to have escaped in a 2013 prison riot that killed six people.

It was unclear if the inmates tried to get to the mainland — historically, most who tried drowned in the attempt — or headed into the hills of the main island. There have long been rumors of escapees living on the island, known as "remontados."

Even some prison employees don't want to leave the main island, located 70 miles (110 kilometers) off the Pacific coast of Nayarit state.

Garduno said 117 of the 411 employees want to continue working on the island, which is scheduled to become a nature reserve and cultural center.

The last 652 inmates were transferred off the island in late February.