The Bay Area has a piece of the nation's fastest growing group of illegal immigrants, but don't assume you know where they come from.

Try India, whose engineers and programmers help power Google and other Silicon Valley companies, whose doctors heal the Bay Area's sick and whose entrepreneurs and venture capitalists have become a force on both sides of the international date line.

The U.S. Department of Homeland Security estimates that 270,000 unauthorized Indian natives live in the United States -- a 125 percent jump since 2000, the largest percentage increase of any nation with more than 100,000 illegal immigrants here.

The number could be as high as 400,000 undocumented Indians, according to Jeffrey Passel, a national immigration expert who works for the Pew Hispanic Center.

The 6.6 million illegal residents from Mexico dwarf the number of undocumented Indians, according to estimates from the federal Office of Immigration Statistics.

Yet, considering the high level of education of many Indians, immigration experts say the federal report hints at a new phenomenon -- a high-skilled undocumented work force to go along with the nation's sizable numbers of low-skilled illegal workers.

If trends continue, within three years India would trail only Mexico, El Salvador and Guatemala as a source of illegal immigration.

Virtually all the Indians entered the United States legally but violated the terms of their visas, say analysts who study the nation's much maligned immigration system.

"How do you get in? You come across the border, or you arrive here with a visa," said Lindsay Lowell, policy director for the Institute for the Study of International Migration at Georgetown University. "Indians aren't going to be walking across the border like Mexicans."

Indians are one of the most affluent ethnic groups in the United States, with a median household income that is 62 percent higher than the figure for all U.S. households.

Santa Clara County has the country's largest Indian-born population and Alameda County ranks fifth among the nation's 3,141 counties, according to 2006 census data. But there is no way to know what share of Bay Area Indian immigrants are illegal.

The Census Bureau does not ask people about immigration status, and the Office of Immigration Statistics report did not provide state or local estimates. Of the 2.5 million people of Indian ancestry living in the United States, about 1 million are not U.S. citizens.

Immigration lawyers say that particularly among Indians, the ups and downs of Silicon Valley's economy since 2001 are one reason why Indians have fallen out of legal status.

"Most are bachelors; the way they get here is they have a job," Gabriel Jack, a San Jose immigration lawyer, said of his Indian clients.

"They come here as professionals, most often in the H-1B program, and given the fluctuations of Silicon Valley, the business climate, these guys lose their jobs. They get laid off or they wager their hands on a start-up coming in," he said. "The problem with the H-1B program is, you can't have any significant time between jobs" without falling out of legal status.

Indians made up 44 percent of H-1B applicants in the 2005-06 fiscal year, five times the number from second-place China, according to federal data.

Because an immigrant's status can be dependent on the status of a spouse, the break-up of a marriage also can create an illegal immigrant.

Among Indians in the United States, "there has been a rapid increase in the divorce rate. If they are on an H-1, maybe the wife is protected and maybe she isn't," said Navneet Chugh, an immigration lawyer whose firm is based in Silicon Valley and Los Angeles. "The guy is an engineer at HP or Cisco, and he goes home on vacation, and his parents say, 'We have a girl for you.' And they get married, and they come here and have all kinds of problems."

Another source is relatives from India who arrive for a visit on a tourist visa and never go home.

"America is a very attractive country; everybody who comes here wants to stay," said Shah Peerally, a Silicon Valley immigration lawyer. "I can tell you right now, there are nearly 1 billion people in India, of which maybe 800 million want to come here."

The United States has deported slightly less than 500 Indians a year in recent years.

Unless Congress reforms the immigration system, "we are going to see this high-skilled, illegal work force emerging," said Frank Bean, director of the Immigration Research Center at UC Irvine.

"From a narrow economic point of view, it might work. From a social justice, fairness point of view, it's a time bomb."

Reach Mike Swift at 408-271-3648 or mswift@mercurynews.com.