Late last month, six Texas men were sentenced to a combined 51 years in prison after an in-depth investigation by the FBI and its law enforcement partners.

Their crime? Human trafficking. A form of modern-day slavery, where human beings -- mostly women and children -- are smuggled across borders, held against their will, forced to work for little or nothing, and often raped and brutalized.

In this case, as in all such cases, the details are hateful: Four Central American women agreed to pay $5,000 -- half up front -- to be smuggled into the U.S. They were taken across the border into Texas, but the gang then demanded more money. When they didn't pay, the women were forced into lives of total servitude. By day, they worked as unpaid domestic servants. At night, they became the gang's sex slaves. All the while, their families were extorted for more money.

How were their captors caught? Two of the women sought help from neighbors. The gang's leader -- Juan Carlos Soto -- found out and ordered his men to kill the pair. The women were raped and beaten, but later let go. They reported their stories to law enforcement authorities, who searched the residence and found the other two women.

The investigation revealed that the gang was smuggling some 100 illegal aliens each week into the U.S. and that the family had been in the human trafficking business since the early '80s. Two alleged members of the gang -- Jose Luis Villa-Zavala and Hector Soto, pictured above -- fled and are wanted by law enforcement.

An isolated case? Sadly, no. Human trafficking today is a multi-billion-dollar-a-year business, involving nearly a million people each year, according to a 2003 U.S. government estimate. Some 18- to 20,000 come into the U.S.

What's the FBI doing about it? Plenty.