More than 20 rough-toothed dolphins have died since Wednesday's beaching by about 70 of the marine mammals, Florida Keys National Marine Sanctuary spokeswoman Cheva Heck said Saturday.
A day before the dolphins swam ashore, the USS Philadelphia had conducted exercises with Navy SEALs off Key West, about 45 miles from Marathon, where the dolphins became stranded.
Navy officials refused to say if the submarine, based at Groton, Conn., used its sonar during the exercise.
Some scientists surmise that loud bursts of sonar, which can be heard for miles in the water, may disorient or scare marine mammals, causing them to surface too quickly and suffer the equivalent of what divers know as the bends — when sudden decompression forms nitrogen bubbles in tissue.
"This is absolutely high priority," said Lt. Cdr. Jensin Sommer, spokeswoman for Norfolk, Va.-based Naval Submarine Forces. "We are looking into this. We want to be good stewards of the environment, and any time there are strandings of marine mammals, we look into the operations and locations of any ships that might have been operating in that area."
Experts are conducting necropsies on the dead dolphins, looking for signs of trauma that could have been inflicted by loud noises.