Doctors tracking swine flu in this state are investigating a new theory: What if it didn’t originate in Mexico but instead had been floating around the border region for months?

Growing evidence in California suggests that early flu cases had no apparent origin in Mexico. Many of the early California victims — including the first two cases — say they hadn’t traveled to Mexico and had no contact with pigs. Some may have fallen ill before the first Mexicans did.

Those cases contradict the conventional understanding of how the strain originated. They could also offer important clues about the future trajectory of the disease. Other cases here and in other states, as well as abroad, have clear links with Mexico.

But the outbreak in California may have a separate origin. “This virus has been circulating around in the population for some time,” said Gilberto Chavez, an epidemiologist with the California Department of Public Health. Its similar symptoms to a standard flu, he said, meant that “any cases that might have been around were probably seen, treated and diagnosed as regular flu.”

Michael Shaw, associate director for laboratory science for the influenza division of the Centers for Disease Control and Prevention, said the flu theoretically could have appeared first in California, but he cautioned against drawing any conclusions since the strain also exhibited genetic characteristics traceable to Eurasia.

The first case discovered in California was a 10-year-old boy in San Diego County, who fell ill with a fever March 30.