DENVER -- The owners of a Denver sushi restaurant are accused of holding their workers in "indentured servitude" and making them work without pay for years, the attorney general said Tuesday.

Young Jo Kwon, 43, and his 41-year-old wife, Jessie Kwon, made two South Korean immigrants work without pay at the Kwon's restaurant former restaurant, Osaka Sushi, the attorney general's office said.

"The 13th Amendment prohibits and outlaws slavery," said FBI agent Michael McPhillips. "These types of cases are essentially modern day slavery."

The Kwons allegedly threatened employees Jaihee Jo Hong and Jong Chul Choi with the revocation of their "sponsorship," which the Kwons claimed would result in the deportation of the employees' families.

Both Hong and Choi are in the United States legally and have the right to work in the U.S., the attorney general's office said.

Hong informed the FBI that Young Jo Kwon claimed ties to the Korean mafia, and Kwon allegedly told Choi "he knew how to hurt someone who did not obey."

The Kwons are accused of depriving Hong of more than $19,000 in overtime wages from 2000 to 2005. Choi worked for four years unpaid and is owed more than $90,000, the attorney general's office said.

The Kwons are also accused of forging employee tax and labor forms.

Attorney Reed Allen, who represents the two employees, said many immigrants, even if in the country legally, fear coming forward.

"Many come from countries where they fear law enforcement, whether or not they are breaking the law," Allen said.

The Kwons have sold their interests in Osaka Sushi, and now own Sushi Moon, located at 6585 Greenwood Plaza Blvd. in Greenwood Village.

Osaka Sushi's current owner has no connection with the investigation.

They were held in Denver County Jail before posting $10,000 bond each. They were arraigned on May 9 and charged with five counts of theft and forgery.

They will appear before the court again on May 30.

Mr. Kwon, through an interpreter, said his attorney advised him not to comment, but did tell 7NEWS the allegations are false.

The investigation was conducted by the Attorney General's Special Prosecution Unit and the FBI's Civil Rights Program.

According to the FBI, human trafficking is a form of modern-day slavery and is a rapidly growing problem in the U.S. It typically involves aliens and economically disadvantaged victims, the FBI said.

Trafficking cases have been identified in a number of industries, including prostitution, exotic dancing, construction, agriculture, restaurants and domestic services. Any legal resident who would like to report possible civil rights violations should contact the Denver division of the FBI at 303-629-7171.

Allen said victims of human trafficking should speak with an immigration attorney.

"There is not only a way out of their situation, there is a way they can stay here if they are willing to help law enforcement prosecute those crimes," he said.

Allen said human trafficking victims can apply for a T-Visa, which gives them a four-year work-visa and the opportunity to apply for a green card.