An ominous new development in the search for a missing Argentine submarine raises concerns it may have plunged off the continental shelf. It has a crew of 44 sailors.

An international agency said it detected an underwater sound that's consistent with an explosion near the submarine's last-known position in the south Atlantic. On Thursday, an organization that monitors underwater nuclear testing said it picked up the sound just hours after the submarine's disappearance last week. The Argentine navy is not saying what made the noise, but is now searching the area it came from.

A relative of one of the 44 crew members of Argentine missing submarine, is comforted outside Argentina's Navy base in Mar del Plata, on the Atlantic coast south of Buenos Aires, on November 23, 2017. Eitan Abramovich / AFP/Getty Images

Relatives of the crew of the San Juan camped out at the Mar del Plata naval base are heartbroken and angry over the news of the apparent explosion. They're waiting for confirmation from the navy, but many now fear the worst, reports CBS News correspondent Adriana Diaz.

An underwater listening station, installed in June, is one of two that picked up the unexpected sound. The international organization that monitors the stations is now taking a closer look at its data to help search crews hone in on where that sound came from.

The San Juan was midway through a 10-day trip from the southern tip of Argentina to its home port just south of Buenos Aires when it lost contact with the navy nine days ago.

Its captain had reported battery failure. The submarine's last known position is near the edge of the continental shelf where the ocean depth drops as deep as 10,000 feet and the water pressure can crush metal.

"If…the submarine, begins an uncontrolled dive, from which it can't recover in very deep water, the hull of submarine could not withstand the pressure," U.S. Naval Institute's Eric Wertheim said.

Experts say the collapse of the submarine hull could have caused that noise.

"If that were the case, that would obviously result in the death of the crew and the destruction of a submarine," Wertheim said. "There's a lot of skilled experienced people out there doing their best and it will be for them to announce when this goes from a search and rescue to a recovery operation."

The U.S. Naval Institute says the ships now heading to that area will be using sonar and other technology to map out the ocean floor. Even if they find the submarine intact, the Argentine navy fears there was only a week's worth of oxygen to sustain the crew and that time has passed.

