Federal investigators Thursday raided five shuttle-van services in Tucson and Phoenix and accused them of knowingly helping to smuggle tens of thousands of illegal immigrants into the U.S. over four years. A total of 49 van operators and alleged smugglers from Mexico and the U.S. were arrested.

The raids exposed a piece of what authorities say is a network of seemingly legitimate businesses that sneak people across the Mexican border.

"The goal of the shuttle operators is to make the aliens look like any other person who would get into a shuttle, like they were going from the airport to their hotel or on a journey around the Southwest," said Matthew Allen, chief of investigations for U.S. Immigration and Customs Enforcement in Arizona, the busiest illegal gateway into the United States.

Investigators said the shuttle-van companies served illegal immigrants who had already slipped into the country by walking dozens of miles through the desert past Border Patrol checkpoints. They climbed aboard the vans in Tucson and were driven to Phoenix.

Although there are no immigration checkpoints on the 115-mile stretch of Interstate 10 from Tucson to Phoenix, the road is patrolled by state and local police. But once they reach Phoenix, illegal immigrants are usually home free. From there, they typically make their way to jobs all over the country.

While the immigrants carried $30 tickets, in reality they generally pay smugglers around $2,500 to take them from the border to their destination in the U.S.

When corrupt van operators are pulled over, they typically claim that they are legitimate transportation services and that it isn't their business to ask about the immigration status of their passengers.

"This case has exposed that for the lie that it is," said John Morton, assistant secretary of homeland security.

Investigators said the five shuttle-van services were never really legitimate businesses at all but were created solely to help smuggle immigrants, and if there were ever nonimmigrant passengers aboard the vehicles, it was rare.

During the investigation, authorities said, they had the front and back doors of the businesses under surveillance. But they would not give any further details about how they built their case or why they believe the companies were knowingly transporting illegal immigrants.

Sergio Rodriguez, owner of Sergio's Shuttle, a Phoenix transportation service accused of working with smugglers, said there has been "a big mistake."