Mechanics performing a post-flight inspection of a United Airlines jet that had just arrived from China on Thursday found a dead body in the nose gear.

San Francisco International Airport spokesman Mike McCarron said the body was discovered shortly after the plane landed from Shanghai, about 8 a.m.

It appeared to be the body of a male, based on the clothing, but the gender was not entirely clear, McCarron said. The San Mateo County coroner's office was investigating.

The person had apparently crawled inside the nose gear's housing intentionally, suggesting an attempted stowaway, he said. "It's not an easily accessible place," the spokesman said.

Stowing away in such a manner "has been attempted, but it's never been successful," he said.

"At altitude there's no air to breathe and it's maybe minus-40 degrees for 12 hours," McCarron said. "You can't survive."

Federal aviation officials said people occasionally try to enter the country without authorization by hiding in a plane's underbelly.

"People think they can make it into a country by hiding in a wheel well," said Federal Aviation Administration spokesman Ian Gregor. "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 it, he said.

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