Greely, Colo. ? Federal agents investigating identity theft arrested immigrants at meat processing plants in six states Tuesday, shutting down production as workers’ relatives scrambled to bring proof they were legally employed.

Immigration and Customs Enforcement officials said an unknown number of workers were arrested on immigration violations and criminal warrants stemming from a nearly yearlong investigation.

ICE chief Julie L. Myers told reporters in Washington that illegal immigrants and others had stolen or bought the identities and Social Security numbers of perhaps hundreds of U.S. citizens and lawful residents. She said some immigrants had genuine U.S. birth certificates.

Myers said the suspects were trying to get jobs with Greeley-based meat processor Swift & Co., which bills itself as an $8 billion business and the world’s second-largest meat processing company.

No charges had been filed against the company, whose president and chief executive, Sam Rovit, issued a news release denying knowingly hiring illegal workers.

Company officials did not immediately return a call Tuesday afternoon.

Since 1997, Swift has used a government pilot program to check Social Security numbers. Company officials have said one shortcoming may be the program’s inability to detect when two people are using the same number.

Hundreds of workers’ relatives gathered outside the plants. One Utah sheriff’s deputy described the scene as a circus.

The Rev. Bernie Schmitz, second from left, of Our Lady of Peace in Greeley, Colo., prays with family members and supporters of Swift & Co. meatpacking plant workers outside the plant in Greeley. Federal agents raided meat processing plants in six states Tuesday and arrested an unknown number of suspected illegal immigrants in an identity theft investigation.

“They’ve got three buses, a bunch of transport vans, a lot of cars and 150 or so agents,” Cache County Chief Deputy David Bennett said.

The United Food and Commercial Workers planned to ask a judge to stop the raids, the union said in a news release, but there was no word on when or where the request would be filed.

ICE officials didn’t notify the sheriff’s department about the raid, Bennett said. “They didn’t ask for our help,” he said. “We were lucky to find out.”

Grand Island Police Chief Steve Lamken said he refused to let his officers take part in the raid.

“When this is all over, we’re still here taking care of our community and if I have a significant part of my population that’s fearful and won’t call us, then that’s not good for our community,” he said.

The nationality of the immigrants was not clear Tuesday night, but the Mexican government released a statement pledging to ensure that any of its citizens in the raid have “their human rights fully respected, and are given all the necessary assistance, orientation and consular protection.”

In a statement, the department asked U.S. authorities to allow Mexican representatives to visit detainees, and announced plans for visits to all six meatpacking plants.

ICE officials said the total number of arrests might not be released until today.