HOUSTON  Raids on 14 illegal bus companies here have shed light on a seedy underground system that transported illegal immigrants all over the country and that sometimes held them captive until their relatives paid exorbitant fares, federal law enforcement officials said Thursday.

Using minivans, the companies were carrying hundreds of illegal immigrants from Mexico to cities across the United States, taking back roads and traveling primarily at night to avoid the authorities, according to criminal complaints filed in Federal District Court. Twenty-two people were arrested earlier this week on charges of using their businesses to transport illegal immigrants.

The bus companies worked exclusively with smuggling operations, officials said. The owners paid commissions of up to $300 for each passenger to smugglers who had brought the immigrants across the Mexican border. Then they held the immigrants in safe houses for days, often under guard, until they loaded them onto vans, according to court documents.

Agents said that at one of the bus companies raided this week, Super Express Van Tours, they found the operators had used pit bulls and armed guards to keep the immigrants from leaving a safe house next door to the office.