The recent prosecution of the owners of the Ewa Plain’s Aloun Farms on forced labor charges appeared to be an isolated case. Now, however, expansion of the FBI investigation involving numerous farms using Thai labor in Hawaii and on the mainland reveals otherwise. What is being called the largest human trafficking case ever prosecuted in the U.S. is an important step in the global effort to eliminate modern slavery.

A federal grand jury in Honolulu has indicted six people associated with a Los Angeles-based contractor, which also is faced with civil lawsuits alleging labor violations and contract breaches with Hawaii farms.

The indictment alleges that about 400 workers were threatened with deportation and economic stress if they did not accept the terms of their employment.

Nearly a decade has passed since more than 100 countries signed onto a protocol in Palermo, Italy, aimed at preventing, suppressing and punishing human trafficking. The Palermo Protocol was aimed mainly at the trafficking of women and children, but it included abuse of other economically exploited workers.

In Thailand, which signed on to the protocol in 2001, workers have been migrating for low-skilled contract work abroad, including the United States. Many have been subjected to forced labor and debt bondage, according to this year’s annual report by the U.S. State Department on human trafficking.

Last week’s indictment says Thai nationals were brought to the U.S. in 2004 and 2005 through a federal agricultural guest worker program. They then were forced to pay high recruitment fees and, having been stripped of their passports and visas, threatened with deportation back to Thailand, where they would continue to face severe problems created by the debts incurred by having used family land as collateral to pay the recruitment fees.

Although Thailand has been slow to make progress in following a comprehensive anti-human trafficking law that went into effect there in 2008, according to the State Department report, one of the six indicted in Honolulu was recently charged in Thailand with recruitment fraud. The Thai Community Development Center in Los Angeles has provided services to victims, indicating a valuable level of cooperation with authorities.

While the Los Angeles company, Global Horizons Manpower Inc., is tied to the alleged abuse, the FBI is trying to determine the extent of other farms’ participation.

Brothers Alec and Mike Sou, owners of Aloun Farms here, have pleaded guilty to wrongdoing and await sentencing in federal court. The Sous reportedly used Global Horizons for a brief period but later decided instead to engage directly in the worker recruitment.

Use of foreign farm labor has become an important element of U.S. agriculture, but the federal program can be abused. The FBI indictment should be recognized throughout the industry as a stern warning.