MAR DEL PLATA, Argentina — For more than a week, as the days passed with little news, relatives of 44 sailors aboard an Argentine Navy submarine that went missing on Nov. 15 hoped for a miracle.

On Thursday morning came the crushing news: An explosion had been recorded deep in the Atlantic Ocean near where the submarine was traveling, only a few hours after the vessel’s last communication. Some relatives fainted. Some screamed at the naval officers. Others sobbed loudly.

There was also anger. Families said the navy had mismanaged the situation by waiting to start a full-scale search and by dangling reports of possible satellite phone calls from the ship, which turned out to be false. Perhaps worst of all, they said, Argentina had let its military degrade to the point of recklessness.

“For 15 years, the navy has been neglected,” said Itatí Leguizamón, the wife of Germán Oscar Suárez, a radar operator on the vessel.

As if to add insult to injury, the explosion came to light only after analysts from the United States government and an international nuclear weapons monitor detected it and told the Argentines. Vessels from the United States, Britain, Brazil and Chile have been combing the seas as part of the search; the Argentine Navy’s four P3-B maritime patrol aircraft have been grounded and unavailable for deployment, according to Jane’s Research.While the Navy did not formally give 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.