MAR DEL PLATA, Argentina — It was a routine mission to interdict illegal fishing boats off the coast of Patagonia.

The first hint of trouble came shortly after 7:30 a.m. on Wednesday, when the captain of the Argentine Navy’s ARA San Juan submarine called his home port to report damage to the vessel’s battery system.

Supervisors on land ordered the vessel to abort its mission and head home. They expected it would do so at “periscope depth” — close enough to the surface to call shore if it was in danger. And last they heard, that is what the submarine was doing.

Then there was only silence.

Now, six days later, the worried families of the 44 crew members know little more than that, despite one of the largest international maritime rescue efforts in modern times.