Was it all a mirage?

Some experts are challenging the claim by Jose Salvador Alvarenga that he miraculously survived 13 months at sea by eating fish, birds and turtles before washing ashore on the remote Marshall Islands 6,500 miles from Mexico.

"I may have some doubts," said Gee Bing, acting secretary of foreign affairs for the Marshall Islands, where Alvarenga washed ashore. He noted that Alvarenga looked thin when he was found, but he could walk on his own and was not as emaciated as might be expected considering how little he had to eat or drink.

Bing said island officials are investigating Alvarenga's story. The Telegraph reported that nobody reported him missing back home in Mexico, but investigators said the algae growth on the side of the boat indicates it had been in the water for some time.

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"It's hard for me to imagine someone surviving 13 months at sea," said Ambassador Tom Armbruster in Majuro. "But it’s also hard to imagine how someone might arrive on Ebon out of the blue. Certainly this guy has had an ordeal, and has been at sea for some time."

Officials were reacting cautiously to the Spanish-speaking man's story while they try to piece together more information.

If true, the man's ordeal would rank among the greatest tales ever of survival at sea.

Mexico's Foreign Relations Department says the man told Mexico’s ambassador to the Philippines, Julio Camarena, that he set out from an area near the coastal town of Tonala in southern Chiapas state, which would mean his journey covered a distance of more than 6,500 miles, if he drifted in a straight line.

Armbruster said the soft-spoken man complained of joint pain Monday and had a limp but was able to walk. He had long hair and a beard, the ambassador said, and rather than appearing emaciated he looked puffy in places, including around his ankles. Otherwise, he added, Alvarenga seemed in reasonable health.

Armbruster, who speaks Spanish, said the survivor told the following story:

He's a native of El Salvador but had lived in Mexico for 15 years and fishes for a man he knows as Willie, catching sharks for 25 pesos ($1.90) per pound.

On Dec. 21, 2012, Alvarenga left Mexico in his 23-foot fiberglass boat for a day’s fishing, accompanied by a teen he knew only as Ezekiel, who was between 15 and 18.

A storm blew the fishermen off course, and soon they were lost and adrift.

"He talked about scooping up little fish that swam alongside the boat and eating them raw," Armbruster said. "He also said he ate birds, and drank birds' blood."

After about a month, Ezekiel died, the survivor told officials.

Alvarenga also talked about eating turtles. Once near Ebon, he swam ashore.

"He thanked God, initially, that he had survived," the ambassador said. "He's very anxious to get back in touch with his employer, and also with the family of Ezekiel. That's his driving motivation at the moment."

In Costa Azul, a fishing hamlet near Tonala, fishing boat owner Villermino Rodriguez Solis, who assumes his son is the "Willie" that Alvarenga referred to, said Alvarenga and a companion had gone missing on Nov. 18, 2012, which would imply the sea odyssey lasted 14½ months.

"Here, his colleagues went out in boats to look for them. They spent four days looking for them," said Villermino, who expressed surprise that Alvarenga had been found alive in the Marshall Islands.

Residents of Costa Azul said they didn't know Alvarenga's real name. He had shown up looking for work years before, but worked from fishing camps along the coast. They knew him only by a nickname, "La Chancha," used to describe heavy-set people. It was clear he was an experienced fisherman, they said.

Armbruster said the man said he had no family in Mexico but he does have three brothers who live in the U.S., although he could not immediately provide officials with contact details. The Mexican government also quoted the man as saying he had no family in Mexico. Bing said he was somewhat skeptical of Alvarenga’s account after meeting with him Monday.

"It does sound like an incredible story, and I'm not sure if I believe his story," Bing said. "When we saw him, he was not really thin compared to other survivors in the past. I may have some doubts. Once we start communicating with where he’s from, we'll be able to find out more information."

Bing said the man had no identification with him, and other details of his story remained sketchy. Camarena said Alvarenga gave his hometown as Garita Palmera, in El Salvador’s Ahuachapan province. Mexico said it was coordinating with the Salvadoran government to provide assistance to the man.

The survivor's vital signs appeared good except that his blood pressure was a bit low, Bing said. After doctors give him the all-clear, Bing said, officials hope to repatriate him to Mexico or whatever country is appropriate.

Erik van Sebille, a Sydney-based oceanographer at the University of New South Wales, said there was a good chance a boat drifting off Mexico’s west coast would eventually be carried by currents to the Marshall Islands. He said such a journey would typically take 18 months to two years depending on the winds and currents, although 13 months was possible.

"The way that the currents in the Pacific work is that there is a very strong westerly current just north of the equator and that basically drives you directly from Mexico all the way toward Indonesia and in the path, you go right over the Marshall Islands," he said.

There have been other cases of people surviving for months adrift in the Pacific. In a case with similarities, three Mexican shark fishermen in 2006 said they were lost at sea for nine months before being rescued near the Marshall Islands. In 1989, four men survived nearly four months in the Pacific Ocean near New Zealand after their multi-hulled boat capsized.

The Associated Press contributed to this report