OMAHA, Neb. — A large federal law enforcement operation conducted Wednesday targeted businesses in Nebraska and Minnesota that officials say knowingly hired — and mistreated — immigrants who are in the U.S. illegally.

The investigative arm of U.S. Immigrations and Customs Enforcement — Homeland Security Investigations — led the operation that saw about a dozen businesses and plants raided and the arrests of 14 business owners and managers and more than 130 workers. Most of the arrests occurred in northern Nebraska and southern Minnesota. Several of the businesses were in O’Neill, Nebraska, a town of about 3,000 about 160 miles (260 kilometers) northwest of Omaha. Officials said they were still looking to take three owners or managers into custody as part of the operation.

Special agent in charge Tracy Cormier described the operation as one of the largest in Homeland Security Investigations’ 15-year history.

“I would say the amount of criminal warrants that are being executed will be one of the largest for HSI,” she said. “I’m not aware of a bigger one.”

Between 350 and 400 federal, state and local law enforcement officers worked together on the arrests, she said.

The focus of the operation is unusual in that it targeted business operators for arrest. Most immigration raids have targeted workers suspected of being in the country illegally.