Mechanics inspecting United Airlines flight 858 discovered the body inside the nose gear well of the Boeing 747 after the plane landed from Shanghai at 7:42 a.m., San Francisco International Airport spokesman Mike McCarron.

The man, who was wearing several layers of clothes and appeared to be Asian and in his 50s, had few obvious injuries, according to San Mateo County Coroner Robert Foucrault. He could have died from a lack of oxygen at altitude, from hypothermia or after being crushed by the landing gear, Foucrault said.

The man had apparently crawled inside the nose gear's wheel well intentionally, suggesting an attempted stowaway, McCarron said.

"It's not an easily accessible place," he said, explaining that few people survive such journeys because the conditions are extremely harsh.

"At altitude, there's no air to breathe and it's maybe minus-40 degrees for 12 hours," McCarron said.

An autopsy was scheduled for Friday to determine the cause of death, Foucrault said.

Federal aviation officials said people occasionally try to enter countries without authorization by hiding in a plane's underbelly. Counting Thursday's victim, the Federal Aviation Administration has tallied 75 similar stowaway attempts on 65 flights worldwide since 1947.

The vast majority — 59 — ended in death, according to FAA spokesman Ian Gregor.

"People think they can make it into a country by hiding in a wheel well," Gregor said. "Almost invariably they get crushed to death, freeze to death, or fall to death."

But there is little U.S. regulators can do to prevent the practice on international flights, he said.

"The security issue is with the origin airport," he said.

The last time someone was found alive after a stowaway flight to the United States was in 2004, when a survivor made it to Miami on a plane from the Dominican Republic, Gregor said.

While TSA works with partners around the world to create minimum standards for airport security, the safety of foreign airports ultimately is in the hands of local authorities, said Christopher White, TSA spokesman.

"How he went into wheel well, that would have to be addressed in Shanghai," said White. "Perimeter security at the Shanghai airport is the responsibility of Shanghai authorities."

Stowaways who survive are usually sent back to their country of origin. That was the case with Fidel Maruhi, who lived through an ascent of 38,000 feet inside the wheel well of a Los Angeles-bound Air France flight originating in Tahiti in 2000.

Maruhi was hospitalized with a body temperature of 79 degrees Fahrenheit — well into the fatal range — after he was found at Los Angeles International Airport. He was treated for four days for frostbite and hypothermia, then sent home.

Sometimes stowaways are allowed to stay after flying illegally into a country.