U.S. immigration authorities and local law enforcement personnel arrested 11 undocumented immigrants Wednesday at a dairy farm in eastern Colorado after a grand jury indicted a score of the farm's employees on charges they used false documents to get hired.

The operation at the Wildcat Dairy in Fort Morgan, about 80 miles from Denver, was the result of a "routine" review of the employment eligibility forms, or I-9s, submitted by the farm's employees, Immigration and Customs Enforcement said.

ICE determined that 53 Wildcat employees - representing 89 percent of the farm's workforce - lacked authorization to work in the United States.

The federal agency conveyed that information to the Morgan County Sheriff's Office and a county grand jury issued indictments last week for 20 Wildcat Dairy employees from Mexico and Central America accused of using forged Social Security and alien registration cards.

Eleven of the indicted workers were apprehended Wednesday and the other nine remain at large. All 20 face charges of criminal impersonation.

Morgan County Sheriff's deputies and Fort Morgan police took part in Wednesday's raid on the farm, ICE said.

The 11 people in custody are being held without bail in Fort Morgan, a town of 28,000 residents that is more than one-third Hispanic, according to 2010 Census figures.

David Marwell, Special Agent in Charge of ICE Homeland Security Investigations in Denver, stressed the importance of keeping government-issued credentials out of the hands of forgers who can use sophisticated equipment to create "high-quality false documents."

The 20 Wildcat employers indicted in Morgan County represent only "the most notable" immigration violations going on at the dairy farm, Marwell said.

"The management of Wildcat Dairy cooperated with the arrest operations" and will not face any charges, the Morgan County Sheriff's Office said in a statement.

Wildcat Dairy produces wheat and corn as well as dairy products and receives federal farm subsidies in the range of $20,000 to $42,000 a year.