The immigrants, mostly young men from poor villages, say they are fleeing religious and political persecution. More than 1,600 Indians have been caught since the influx began here early last year, while an undetermined number, perhaps thousands, are believed to have sneaked through undetected, according to U.S. border authorities.

Hundreds have been released on their own recognizance or after posting bond. They catch buses or go to local Indian-run motels before flying north for the final leg of their months-long journeys.

"It was long … dangerous, very dangerous," said one young man wearing a turban outside the bus station in the Rio Grande Valley town of Harlingen.

The Indian migration in some ways mirrors the journeys of previous waves of immigrants from far-flung places, such as China and Brazil, who have illegally crossed the U.S. border here. But the suddenness and still-undetermined cause of the Indian migration baffles many border authorities and judges.

The trend has caught the attention of anti-terrorism officials because of the pipeline's efficiency in delivering to America's doorstep large numbers of people from a troubled region. Authorities interview the immigrants, most of whom arrive with no documents, to ensure that people from neighboringPakistan or Middle Eastern countries are not slipping through.

There is no evidence that terrorists are using the smuggling pipeline, FBI and Department of Homeland Security officials said.

The influx shows signs of accelerating: About 650 Indians were arrested in southern Texas in the last three months of 2010 alone. Indians are now the largest group of immigrants other than Latin Americans being caught at the Southwest border.

The migration is the "most significant" human-smuggling trend being tracked by U.S. authorities, said Kumar Kibble, deputy director of Immigration and Customs Enforcement, or ICE. In 2009, the Border Patrol arrested only 99 Indians along the entire Southwest border.

"It's a dramatic increase," Kibble said. "We do want to monitor these pipelines and shut them down because it is a vulnerability. They could either knowingly or unknowingly smuggle people into the U.S. that pose a national security threat."

Most of the immigrants say they are from the Punjab or Gujarat states. They are largely Sikhs who say they face religious persecution, or members of the Bharatiya Janata Party who say they are targeted for beatings by members of the National Congress Party.

But analysts and human rights monitors say political conditions in India don't explain the migration. There is no evidence of the kind of persecution that would prompt a mass exodus, they say, and Sikhs haven't been targets since the 1980s. The prime minister of India, Manmohan Singh, is a Sikh.

"There is no reason to believe these claims have any truth to them," said Sumit Ganguly, a political science professor and director of the India Studies Program at Indiana University.

Some authorities think the immigrants are simply seeking economic opportunities and are willing to pay $12,000 to $20,000 to groups that smuggle them to staging grounds in northern Mexico. Kibble said smugglers may have shifted to the Southwest after ICE dismantled visa fraud rings that brought Indians to the Northeast.

Many Indians begin their journey by flying from Mumbai to Dubai, then to South American countries such as Ecuador or Venezuela, according to authorities and immigration attorneys. Guatemala has emerged as the key transit hub into Mexico, they said. The roundabout journeys are necessary because Mexico requires visas for Indians.

They sneak across the dangerous Guatemala-Mexico border and take buses or private vehicles to the closest U.S.-Mexico border. Mexican organized crime groups are suspected of being involved either in running the operations or in charging groups tolls to pass through their territory.

The Indians usually wade across the Rio Grande, and then are shuttled from stash houses to transportation rings that take them north. David Aguilar, deputy commissioner of Customs and Border Protection, an agency within the Department of Homeland Security, said he believed a high percentage were caught as soon as they crossed the river.

"We very intensely interview, look at their backgrounds, check them against any watch list," Aguilar said, adding that although India is not considered a "special interest" source country for terrorists, the undocumented immigrants are scrutinized as if it were.

The detainees eventually claim asylum. In January, immigration court calendars at the area's two main detention facilities were full of the common Indian surnames Patel and Singh, and attorneys and judges struggled to keep up. Some attorneys had failed to file the necessary forms; interpreters were not always available. Judge Keith Hunsucker said more immigration judges would soon be assigned to handle the increased workload.

Many detained immigrants clear the first hurdle toward a full asylum hearing by convincing asylum officers they have a "credible fear" of persecution if they return to India. They can then post a bond and move anywhere in the United States as long as they agree to appear for their next court date.

Not all show up, however. "That's why I won't take their cases anymore," said Cathy Potter, a local immigration attorney who helped about 20 Indians get freed on bond last year. "It undermines my credibility. I don't want anything to do with this."

It is not clear how many Indians have been granted asylum or deported; immigration officials did not fulfill requests for that information. Judges and attorneys appear to be toughening up, however. Bond amounts have risen sharply in recent months, and attorneys say asylum claims are increasingly being rejected.

Judge William Peterson raised doubts during a recent hearing when a 27-year-old Punjabi woman said she had been beaten and raped, her sari ripped off by several attackers. The petite woman, her long hair in a ponytail, said she was targeted because her husband was a driver for National Congress Party officials.

"I haven't heard you tell me anything that you did on behalf of the party that would irritate these people," Peterson said at the hearing held by video conference.

"We used to give help to the poor. They did not like that," she said. Peterson rejected her claim for a finding of "credible fear," deeming her story inconsistent with statements she had made to an asylum officer. "They're going to kill me. They're going to rape me," she pleaded, wiping away a tear.

But hundreds of immigrants have persuaded asylum officers and judges to grant credible-fear findings, clearing the way for bond hearings.

Hunsucker, an immigration judge at the Port Isabel Detention Center near Brownsville, set bond amounts ranging from $15,000 to $40,000 for 10 Indians one recent morning.

Most said they had relatives or friends in the U.S. willing to sponsor them, though the judge raised concerns about some. In one case, a young man said his sponsor was his cousin, a woman. But the faxed identification document of the cousin showed a picture of a man with a beard. The bond was set at $15,000.

Once released, the immigrants are transported to the Greyhound bus station in downtown Harlingen. One recent evening, 10 Indians crowded around pay telephones and the bus counter, struggling with limited English skills to arrange travel.

One young man paid for a $204, two-day bus ride to New York City. When the clerk asked his name, he handed over his detention center ID wristband.

A young man wearing a turban asked the clerk for information on the next bus to Indiana. He spoke broken English and later tried to provide details about his journey, but other immigrants nudged him to keep quiet. The trip was worth it, he said, adding, "I'm happy, because it's safe" in the U.S.

Outside, motel operators offered to shuttle the men to their nearby quarters. Shoving matches between motel operators have broken out in recent weeks as they compete to fill their $44-per-night rooms with immigrants.

The Indians are largely unseen in the towns along the Rio Grande Valley, where they disappear into detention centers, stash houses or motel rooms. Some Sikhs have been confronted by locals alarmed by the sight of people wearing turbans, motel workers say.

Federal agents investigating human-smuggling rings have visited at least one motel, America's Best Value Inn in Raymondville, workers said. General Manager Kevin Patel denied any wrongdoing.

He houses about 20 Indians per week, he said, shuttling them to and from the bus station and printing out airline boarding passes. He serves them meals in his motel apartment, often the first Indian food they've had in months, he said.

One recent guest, Bharat Panchal, 37, said he was released from detention in late January after friends posted his $20,000 bond. India had become dangerous, he said, because of political unrest in his home state of Gujarat. He was flying later that day to Los Angeles to live with a friend, he said.

Patel said the sudden appearance of Indian immigrants in southern Texas baffled him.

"When they first showed up, I scratched my head a little bit," Patel said. But he has opened his doors and makes the immigrants feel at home.

"They need a place to stay," he said. "They need food. They speak my language, so of course, as a human being, I can help them out."

richard.marosi@latimes.com

abecker@cironline.org

This report is published in cooperation with the nonprofit Center for Investigative Reporting in Berkeley, where Becker is a staff reporter.