Immigration officers are investigating workplaces in every state in the U.S. to check whether they are hiring illegal workers.

Immigration and Customs Enforcement told members of Congress in an e-mail Wednesday morning that it is beginning audits of documents that employees fill out when they are hired. The documents are known as I-9's.

President Barack Obama has said his administration's strategy for stemming illegal immigration is focusing on employers who hire illegal workers.

The Bush administration was criticized for raiding businesses and arresting workers but not doing enough to go after the employers who hire them.