MEXICO CITY — No one has identified ground zero in the swine flu epidemic. Just where or when the new strain of influenza first jumped from a pig and began infecting people is a scientific mystery — one that a group of flu detectives is determined to solve.

Scientists are returning next week to La Gloria, a pig-farming village in the Veracruz mountains where Mexico's earliest confirmed case of swine flu was identified. They hope to learn where the epidemic began by taking fresh blood samples from villagers and pigs, and looking for antibodies that could suggest exposure to previous swine flu infections.

Some experts say it's pointless to worry about what happened in La Gloria now that the swine flu virus has spread around the world. But others argue that a thorough investigation could be key to preventing future epidemics.

And Mexico has another reason to care: If it can somehow rule out the possibility that La Gloria's pigs infected humans, then it can tell the world it wasn't to blame for the epidemic — that the never-before-seen H1N1 swine flu virus came from somewhere else.

More than half of La Gloria's 3,000 residents fell ill with flu symptoms weeks before the new virus was identified. Many found it hard to breathe, burned with fever and ached all over. About 450 of the sickest residents were diagnosed with acute respiratory infections and sent home with antibiotics and masks.

Mexican health officials initially downplayed the outbreak, saying the villagers suffered from regular flu. A 5-year-old boy was the only confirmed swine flu case among 43 villagers whose mucous samples were taken in early April. By then, most other villagers had recovered, and the virus was gone from their systems.

But some disease experts suspect swine flu was circulating more widely in La Gloria.

"I cannot understand it. I could almost bet that there were more infections related to this virus" in La Gloria, Dr. Carlos Arias told The Associated Press. Arias is leading a group of flu detectives from the Biotechnology Institute and the veterinary school of the National Autonomous University of Mexico back to the village at the invitation of the Veracruz state government.

Pigs — like people — get the flu, usually over the winter months. This new swine flu virus is unusual in that it also has infected humans and at this point has become a full-blown human flu.