TIJUANA, Mexico — It was not the first time Robert L. Brownell Jr. had seen a dead vaquita, the rare and endangered porpoise that was lying on the stainless-steel necropsy table inside the Tijuana Zoo on Monday. But it might well be one of the last.

Mr. Brownell, a senior scientist at the National Oceanic and Atmospheric Administration, had in effect discovered the porpoise, finding the first full, dead specimen in 1966. The world’s smallest member of the cetacean grouping, which includes whales and dolphins, the vaquita was the most recent cetacean to be recognized by modern science.

Now it may well become the latest to go extinct.

A high-level, bilateral panel of Mexican and American scientists met this week and is expected to announce that it believes efforts to save the animal have, essentially, failed. That announcement would mean that the only hope for the vaquita’s recovery would be to capture the surviving animals, if any can be found. Some of the scientists involved think the surviving vaquitas now number as few as two or three, and the latest two vaquitas found dead could even be the last ones — though it could take years to confirm that.

The vaquita has been described, by some of the few people who have seen one of the elusive porpoises alive, as the smiley-faced sea panda because of the dark outlines around its mouth and the black circles around its eyes. Its plight has drawn in celebrities like Miley Cyrus and Leonardo DiCaprio, and spurred cooperation among scientists and environmentalists, north and south of the border, in and out of government.