While the Navy did not formally given up hope of finding the crew, relatives began referring to their loved ones in the past tense. If the sailors perished, it would be the deadliest submarine catastrophe since the sinking of the Kursk — a Russian vessel brought down by a misfired weapon in 2000 — and the Argentine military’s largest loss of life since the Falklands War of 1982.
The disappearance and likely loss of the vessel, the San Juan, could turn out to be the greatest national tragedy to unfold under President Mauricio Macri, who came into office nearly two years ago vowing to invest in Argentina’s underfunded armed forces.
Even before the latest news, frustration at Mr. Macri had been mounting.
“Instead of spending on other matters, why don’t you spend on something truly important, like the life of all our relatives,” an unidentified woman asked Mr. Macri on Monday when he traveled to the resort city of Mar del Plata, according to a video of the meeting that was posted online.
Using submarines that have been in commission since the early 1980s “is taking a gamble on the life of our people,” she said.
Argentina has spent less than several of its neighbors on defense since the end of military rule in 1983. Despite Mr. Macri’s promises, the effort to repair and replace the country’s aging planes and ships is in a nascent phase.
Last year, Argentina spent about 1 percent of its gross domestic product on the military, lagging behind neighbors like Chile, which spent 1.9 percent, and Brazil, which spent 1.3 percent, according to the Stockholm International Peace Research Institute.
“Argentina’s armed forces began to atrophy after the end of the military rule,” said Dan Wasserbly, the Americas editor of IHS Jane’s, a defense-industry publication. “It’s been a pretty long-running trend where they have talked about investing, adding resources and building up the readiness of a once-formidable military, but they haven’t been able to do that.”
The San Juan, a German-made diesel-electric vessel, was built in 1983 and first put into commission in 1985. It was put back into service in 2014 after a retrofit, and had been scheduled to return on Sunday to its home base at Mar del Plata, about 250 miles south of Buenos Aires.
Even as a multinational search effort combed the seas last weekend, braving stormy weather and 22-foot waves, analysts at the Comprehensive Nuclear Test Ban Treaty Organization in Vienna began considering whether the steady stream of information from monitors that track sound and earth movements across the world may have picked up clues about the missing vessel.
The organization, which is usually focused on picking up signs of nuclear tests, undertook the analysis on its own initiative.
Mario Zampolli, a hydroacoustic engineer at the organization, said he and his colleagues began poring over data from the two nearest sensors to the search area over the weekend. Picking up evidence of a relatively small explosion in the ocean requires manual analysis of data and custom-designed software, Mr. Zampolli said in an interview.
“We try to determine that it doesn’t come from a natural event such as volcanic activities, whales or an earthquake,” he said. The organization shared its findings with the Argentine ambassador in Vienna on Thursday, after concluding that it was “similar to other water explosions observed previously.”
That finding was broadly in line with an assessment the United States shared with Argentina on Tuesday night.
Cmdr. Erik Reynolds, a spokesman for the United States Navy, said American analysts had ascertained that a sound registered in the ocean off the coast of Patagonia was a “hydroacoustic anomaly” that had not been caused by natural events.
“That was not a natural sound you hear in an ocean environment,” he said.
After piecing the two assessments together, and finding that they both pointed to an explosion in the area where the submarine was known to have been, Argentine military officials decided to break the news to relatives. Minutes later, a forlorn-looking navy spokesman, Capt. Enrique Balbi, addressed reporters.
Captain Balbi said there was no evidence that a battery malfunction, which the captain of the San Juan reported shortly before the submarine went silent on Nov. 15, was related to the explosion. The Argentine Navy disclosed the battery problems, which it characterized as a routine mishap, on Monday.
That piece of sobering news soured the mood here after a weekend that had seemed more hopeful. On Saturday, Argentina’s defense minister said there were records of satellite phone calls placed from the submarine that day, suggesting the crew was alive. The following day, however, with little explanation, the Navy had to acknowledge that no such calls were tracked.
Still, the navy defended its response and the condition of the vessel. “The search and rescue plan went ahead according to schedule,” Capt. Balbi said on Thursday night, adding, “The age of these units does not imply their obsolescence.”
Argentine and outside experts have said that even if the San Juan was intact, its crew would probably have only enough oxygen to survive seven to 10 days.
On Thursday, as a sunny morning gave way to a chilly, cloudy afternoon, about 25 people gathered outside the fence of the naval base to pray a rosary. A few friends and relatives of crew members said they were still holding on to a glimmer of hope. But for most, grief was starting to take hold.
“He was my first love,” said Jesica Gopar, the wife of a crew member, Fernando Santilli. “If he can somehow hear me out there, all I can say is I love him and that his son will have a bright future because we have many people who are helping us.”
Ms. Gopar said she had largely stayed away from the base in recent days. But on Thursday she woke up “with the feeling that something had happened.” So she drew up a sign to honor Mr. Santilli and his comrades and headed to the base to add it to the messages of hope and flags pinned up on the outer fence.
“I came to put up a sign, only to find out I am a widow,” Ms. Gopar said.
An earlier version of this article misidentified the agency that supplied corroborating information about an explosion near an Argentine submarine. It was the Comprehensive Nuclear Test Ban Treaty Organization, not the International Atomic Energy Agency.
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