DENVER -- Two and a half years after a series of high-profile raids to round up illegal workers in the United States, an advocacy group issued a report concluding the raids were ineffective and in some cases, unconstitutional.

The United Food and Commercial Workers International Union compiled the report after holding public hearings and individual interviews.

The union has also filed a lawsuit on behalf of some of the workers who claim they were illegally detained.

The Swift Meat Packing plant in Greeley was one of six that was raided on Dec. 12, 2006.

Immigration agents were looking for workers who were in the country illegally and criminally charges were filed against some who were accused of falsifying documents.

The very critical report concluded, "Thousands of workers at each plant were rounded up, detained and criminalized for doing nothing more than reporting to work."

Union President Joseph Hansen told reporters, "Agents trampled on the constitutional rights of U.S. citizens."

Former Colorado Congressman Tom Tancredo told 7NEWS, the detainments were "a small price to pay to enforce the law."

The Immigration and Customs Enforcement Agency released a statement defending the raids.

"We do so professionally and humanely, and with an acute awareness of the impact that enforcement has on the individuals we encounter."

The union plans to submit the report to Congress and ask for more oversight of ICE and employers who hire illegal workers.