The hope that arose from seven failed satellite calls detected on Saturday morning, which Argentina's defence ministry said could be from the missing submarine, is also dimming fast.
The signals were received at 10:52 am (1352 GMT) and 3:42 pm (1842 GMT), but they did not lock in, thus preventing a full connection.
Two vessels searching for the submarine heard a "noise" at a depth of about 656 feet, said navy spokesman Enrique Balbi from Buenos Aires.
There is a feeling of "cautious enthusiasm", naval expert Fernando Morales told C5N television.
The naval commander said that the submarine had been asked to cut short its mission, which was originally due to last until Monday, and go directly to Mar del Plata.
The Southern Command of the United States (SOUTHCOM), based in Miami Florida, reported that a US Military aircraft with a crew of 21 members will depart from Jacksonville, the northern part of the state of Florida, and is expected to arrive this Sunday in the South American country.
The satellite call attempts were registered between 9am and 3pm on Saturday, lasting between four and 36 seconds, but there was no voice contact.
Over the weekend, IARU Region 2 News Editor Joaquín Solana, XE1R, issued a list of marine frequencies, suggesting that radio amateurs and SWLs listen for any signals that could be related to the missing San Juan.
Rescuers are focusing on an ocean patch about 300 kilometers in diameter, radiating from the last point of contact.More news: Apple delays HomePod release until next early 2018
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At the entrance of the base, locals hung signs with messages in support of the crew members and their families on a chain-link fence.
The US Navy has deployed two unmanned underwater vehicles that use a sonar system to create an image of large sections of the sea floor. A Navy inquiry said the cause could not be definitively determined.
Balbi said it was following the northward course the submarine would have taken toward Mar del Plata.
"They've got to be afloat".
Among those on board is Argentina's first female submarine officer, 35-year-old weapons officer Eliana Krawczyk.
"We do not exclude any hypothesis", repeated the navy in its press releases.
Authorities say the position is in line with the path the sub would have taken to reach the base in Mar del Plata as planned and a U.S. military aircraft has been sent to check the area. Built in Germany by Nordseewerke, it underwent mid-life maintenance in 2008 in Argentina.
The Argentine Navy has said that they do not believe the malfunction was the reason the submarine has been missing. Argentine authorities received a few blips of hope in their effort to find a three-decade old submarine - and 44 crew members - that suddenly stopped communicating during a routine mission on Wednesday.