BUENOS AIRES, Argentina — Argentina’s Navy said Thursday it is no longer looking for survivors among the 44 sailors aboard a submarine missing for 15 days, though a multinational operation will continue searching for the vessel.

Hopes of finding survivors had already dimmed because experts said the crew had only enough oxygen to last 7 to 10 days if the sub remained intact under the sea. The Navy also had said an explosion was detected near the time and place where the ARA San Juan made its last contact with shore Nov. 15.

The San Juan, a German-built diesel-electric TR-1700 class submarine, was commissioned in the 1980s and was most recently refitted in 2014.

Navy spokesman Enrique Balbi said the rescue mission had “extended for more than twice what is estimated for a rescue.”

“We’ve had 28 ships, nine aircraft, 4,000 people involved, 18 countries supporting,” he told reporters. “Despite the magnitude of these efforts, we’ve been unable to find the submarine.”

Check out these stories on the multitude of technology used in the effort to find the submarine:

Associated Press writer Almudena Calatrava in Buenos Aires and Julie Watson in San Diego contributed to this report.