SAN ANTONIO, Texas - Nothing seemed unusual about 115 Westhaven Place, at least not at first. The tiny ranch-style house had the same low-slung roof and patchy front yard as others on the sleepy cul-de-sac. Its window blinds stayed tightly shut against the Texas heat.

But one morning last month, a helicopter roared overhead as federal agents wielding battering rams smashed the front door and a window and led out 11 undocumented immigrants wearing only underwear. Two men were arrested and charged with human smuggling.

Federal officials allege that the dwelling was a stash house, a vital part of the lucrative smuggling industry along the U.S.-Mexico border. After sneaking immigrants and drugs across the border, smugglers often head straight to these hideouts. They collect payments - which can total thousands of dollars - and essentially imprison the immigrants there temporarily while planning for their next destination.

"They're a critical stage of the whole smuggling process," said Jerry Robinette, a former special agent in charge of Immigration and Customs Enforcement's Homeland Security Investigations in San Antonio. "They're definitely a strategic place."

Smugglers have used stash houses to move people and drugs for years, often in small groups to minimize the risk of being caught. But experts say these routes are increasingly controlled by criminal drug cartels. This year, far fewer immigrants are crossing, potentially cutting into the cartels' profits.

Federal officials said they have discovered 259 stash houses on the southwestern border this fiscal year, which ends in September. In one of the largest sectors, Texas's Rio Grande Valley, officials busted 77 stash houses this year, compared with 86 last year. They caught 880 immigrants in those stash houses, down from more than 1,300 last year.

In July, authorities stumbled upon a mobile home in Laredowhere 17 undocumented immigrants and nearly four pounds of marijuana had been hidden, and they rescued another group of immigrants farther east, in Brooks County. Some had hiked for hours under the hot sun and then bounced among multiple stash houses before being loaded into a trailer that was cooled to 38 degrees.

One witness, terrified because she could not breathe in the frigid truck, said a stash-house boss warned her not to speak to authorities if they were caught or "something would happen to her."

Robin Reineke, executive director of the Tucson, Arizona-based Colibri Center for Human Rights, said stash houses will persist as long as "folks don't have a safe and legal pathway to come to the United States to fill the jobs."

Federal and state officials urge residents to report suspected stash houses to the Department of Homeland Security's anonymous tip lines. Telltale signs include different cars and trucks coming and going as well as neighbors who rarely go outside.

"We really do rely on the public, folks in the community, to provide us leads," said Shane Folden, the special agent in charge of ICE's Homeland Security Investigations in San Antonio. "At the end of the day, these are criminals. These are folks that harm other people. Sometimes, other people die because of them."

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In 2014, Texas started offering cash rewards of up to $2,500 to people who report stash houses. Police, who also have beefed up their presence along the border, have received dozens of tips and issued seven rewards, most recently to someone whose information led to the recovery of 22 suspected undocumented immigrants in Laredo in June.

"We're making it more difficult," said Katherine Cesinger, spokeswoman for the Texas Department of Public Safety. "It disrupts their whole business model."

But many residents say they fear the drug cartels that are believed to run the smuggling networks.

"I don't mess with people like that," said one man, a truck driver, as he drank a beer in a lawn chair near the house on Westhaven Place, which was boarded up and ringed by yellow police tape after the raid.

Authorities say that stash houses are often filthy and dilapidated and that immigrants are held there without sufficient food, water or sanitary facilities. Women have been raped at stash houses, and migrants have been beaten, tortured and held hostage until their families pay a ransom.

In some cases, agents said, rival gangs have kidnapped immigrants from stash houses to extort money from their families.

"There's some serious, serious stuff that happens in these homes," Robinette said. "The longer they stay there, the more volatile the situation gets."

The house on Westhaven Place was raided hours after a stifling tractor-trailer packed with migrants pulled into a Walmart parking lot several miles away. Dozens fled the truck as soon as the doors were opened, witnesses later told authorities. But 10 people died of excessive heat and dehydration. More than two dozen others were injured.

The truck driver, who has been indicted on charges related to the deaths, is accused of picking up at least some of the immigrants after they were held in stash houses closer to the border, according to court records. The driver has pleaded not guilty.

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On Westhaven Place, in a neighborhood of tightly packed single-family homes and oak trees near Lackland Air Force Base, the smugglers allegedly forced the men to strip down to their underwear so they would not flee the house without payment or expose their operation to neighbors.

The two men who were arrested, Ismael Benavidez, a U.S. citizen, and Jesus Antonio Martinez, a Mexican citizen in the United States illegally, told federal agents that the men were fed and cared for, according to court papers.

Agents wrote in affidavits that Benavidez, who was arrested at a gas station near the house, said Martinez paid him $200 a week to take care of the immigrants, plus $100 every time he picked up a money order - payment for smuggling fees - at Western Union or MoneyGram.

Benavidez and Martinez are charged with illegally harboring immigrants for financial gain. Their attorneys did not respond to requests for comment.

The immigrants said they slipped across the border July 16 and were later picked up in an empty parking lot and ferried to the house. J.G., a citizen of Mexico, told authorities that he had paid $3,500 in smuggling fees but still owed $2,500 when officials arrested them.

The discovery of the stash house on the cul-de-sac startled many neighbors, some who've lived there for years.

An elderly man who lives to the right of the stash house said he never heard anything, according to his daughter, Kelly Green. She said the neighborhood used to be filled with close-knit military families whose kids raced bicycles and ate peanut butter sandwiches and camped in one another's yards - "the perfect quintessential place to grow up."

Others said the neighborhood had changed since Kelly Air Force Base shut down and people lost their jobs or moved away.

Juan Guzman, 70, a retired machinist who is the other immediate neighbor, said the house had a series of renters since the death of its owner a few years ago. For a while, he said, alleged gang members rented there, leading to fights, loud parties and at least one shooting.

He said he complained to the real estate agency, which briefly rented the property to a young woman.

After she disappeared, the grass grew too high and the house was too quiet. Four months ago, a gray-haired man knocked on Guzman's door and asked him in Spanish for a baseball cap. As Guzman answered him, the man apparently heard something, abruptly turned and ran off.

Yuvianna De Lellis, 29, a mother of four who lives across the street, said she unwittingly asked people she saw pulling into the stash house the Friday before the raid for help moving a piece of furniture. Three men obliged.

"I had no idea," she said. "I'm just very thankful they kept to themselves."