MAR DEL PLATA, Argentina (AP) — Argentine families of 44 crew members aboard a submarine that has been lost in the South Atlantic for seven days are growing increasingly distressed as experts say that the crew might be reaching a critical period of low oxygen Wednesday.

The ARA San Juan went missing Nov. 15 when it was sailing from the extreme southern port of Ushuaia to the city of Mar del Plata, about 250 miles (400 kilometers) southeast of Buenos Aires. The Argentine navy and outside experts worry that oxygen for the crew would only last seven to 10 days if the sub is intact but submerged. Authorities still do not know if the sub rose to the surface to replenish oxygen and charge batteries.

The German-built diesel-electric TR-1700 class submarine was set to arrive Monday to a naval base in Mar del Plata, where local residents have arrived bearing blue-and-white Argentine flags and messages of support for relatives of the crew anxiously waiting for news.

More than a dozen international airplanes and ships have joined the maritime search despite stormy weather that has caused powerful waves of more than 20 feet (6 meters).

Hopes were lifted after brief satellite calls were received and when sounds were detected deep in the South Atlantic. But experts later determined that neither was from the missing sub. A U.S. Navy P-8 Poseidon aircraft later spotted flares, and a life raft was found in the search area, but authorities said that they did not come from the missing submarine.

The false alarms have rattled nerves among distraught family members. As the search enters a critical phase, some have begun to complain that the Argentine navy responded too late.

“They took two days to accept help because they minimized the situation,” Federico Ibanez, the brother of 36-year-old submarine crew member Cristian Ibanez, told The Associated Press.

The navy has said the submarine reported a battery failure before it went missing as it journeyed to the navy base in Mar del Plata. Authorities have no specific details of the problem.

“I feel like authorities let too much time pass by and decisions were taken late,” Ibanez’s sister, Elena Alfaro, said outside the base. “And yet, I still carry some hope.”