More than a dozen farms in Hawaii were involved in what the FBI is calling the largest human trafficking case ever prosecuted in the U.S., according to a sweeping federal indictment unsealed yesterday.

Six people accused in the case are all associated with Global Horizons Manpower Inc., a Los Angeles-based contractor already embroiled in several civil and class-action lawsuits that allege labor violations and contract breaches with local farms.

About 400 workers from Thailand circulated through island farms — some were sent to the mainland — through Global Horizons from May 2004 through September 2005. The workers were threatened with deportation and economic stress if they did not work, the indictment said.

“This is the largest human trafficking case ever charged in United States history,” FBI Special Agent Tom Simon said, referring to the number of victims.

“In the old days, they used to keep slaves in their place with whips and chains,” Simon said in an interview. “Today, it is done with economic threats and intimidation.”

Global Horizons recruits workers from foreign countries through the U.S. Department of Labor H-2A guest worker program to work on U.S. farms, including properties in four Hawaii counties.

Company officials targeted impoverished Thai nationals, who earned about $1,000 a year in their home country, the indictment said. They were promised three years of employment and high wages ranging from $8.72 an hour to $2,000 a month for eight hours of work a day, according to the indictment.

Company officials then required the workers to pay substantial recruitment fees, from $9,500 to $21,000, to get jobs, the indictment said. The company also secured deeds to family land in Thailand as collateral.

Unable to pay that debt, workers faced threats of deportation and property foreclosures if they did not work, the indictment said. The company confiscated their passports and visas to restrict their movement while in the U.S.

Global Horizon officials allegedly profited from the contracts, receiving portions of the recruitment fees as well as commissions Hawaii farms paid to get workers.

The FBI said some of the workers have gone back to Thailand, while others remain in Hawaii and the mainland, including Washington state.

Some of the workers contacted nongovernmental organizations and immigration attorneys, who then got the FBI involved. Federal agents investigated the case in 2008, but took two years to bring the case to court because of its international implications, Simon said.

There were 14 agricultural properties that used the services of these workers, but none has been charged in this case.

“The FBI is attempting to learn the extent that these farms were aware of the forced labor conditions of its workers,” Simon said. “With few exceptions, the farms have been cooperating with our investigation.”

One of those exceptions, federal investigators confirmed, is Aloun Farms. Proprietors of the Ewa farm face years in federal prison for conspiring to commit forced labor in connection with the importation of 44 farm workers from Thailand in 2004.

Brothers Alec and Mike Sou, owners of Aloun Farms, pleaded guilty in January and will be sentenced next week.

The indictment also said Global Horizon confined workers to the property of the now-defunct Maui Pineapple Farm, posting guards to prevent the workers from running away.

Two of the defendants strung yellow tape around the Maui Pineapple housing area and installed bells on string lines in the woods to alert guards of escape attempts, according to the indictment.

The defendants are scheduled to appear Thursday in U.S. District Court in Hawaii.

Global Horizons has been in and out of court fighting labor complaints throughout the U.S. and Hawaii.

On Tuesday, the Kauai Coffee Co. filed a lawsuit against Global Horizons. The two companies entered into an H-2A agreement for workers from Thailand in 2004.

Since 2006, 17 people have filed worker discrimination complaints against Kauai Coffee Co. through the U.S. Equal Employment Opportunity Commission, 15 of whom filed through the Honolulu office.

Kauai Coffee denies the allegations, but has had to defend itself against the EEOC charges, racking up attorneys’ fees and associated court costs. Kauai Coffee claims in its lawsuit that Global Horizons should be defending the charges on its behalf.

An attorney representing Kauai Coffee declined comment yesterday, citing the continuing case.

In 2008, Florida-based Del Monte Fresh Produce Inc., which operates in Hawaii, filed a similar lawsuit in Circuit Court against Global Horizons, alleging a breach of contract. Del Monte faces 28 charges from the U.S. EEOC.

Del Monte alleges that Global Horizons breached a 2003 contract by “failing to pay the temporary production workers.”

“Subsequent to Global Horizons’ conduct, Del Monte Hawaii had a difficult time obtaining employees for that facility through other temporary agencies and has had its reputation injured due to the allegations of discrimination,” the lawsuit states.

In May 2006, the U.S. Department of Labor ordered Global Horizons to pay about $300,000 in fines and back wages for deceiving and underpaying 88 Thai workers hired to harvest onions and pineapples in Hawaii.