WASHINGTON -- The Obama administration launched investigations of hundreds of businesses around the U.S. Wednesday as part of its strategy to focus immigration enforcement on the employers who hire illegal workers.

Immigration and Customs Enforcement has begun notifying businesses of plans to audit their I-9 forms -- employment eligibility documents that employers fill out for every worker -- the agency told members of Congress in an e-mail Wednesday.

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

The Bush administration was criticized for deploying armed agents to raid businesses and arrest workers suspected to be working illegally. Critics said the Bush administration did not do enough to go after the employers.

Immigration officers served "Notices of Inspection" to 652 businesses, the Homeland Security Department said. By comparison, 503 such notices were issued to businesses last year, the agency said. Businesses were chosen for inspections based on leads and other investigative work, Immigration and Customs Enforcement said.

Employers are required to keep the I-9 forms and must check the authenticity of documents provided by the employee. The Homeland Security Department said it would not release the names or locations of the businesses that are being audited because of the ongoing investigations.

"ICE is committed to establishing a meaningful I-9 inspection program to promote compliance with the law," John Morton, Immigration and Customs Enforcement director, said in a statement. 

"This nationwide effort is a first step in ICE's longterm strategy to address and deter illegal employment."

The Obama administration has been trying to build its credibility on immigration enforcement to boost the chances of passing an immigration reform bill in Congress. The administration has doubted whether it has enough votes right now to pass immigration reform. But some members of Congress emerged from a meeting with Obama last week saying immigration reform could be done by the end of the year or early next year.

The I-9 audits are certain to cause concern among employers who have complained that identifying illegal workers is fraught with problems, from recognizing fake identity documents made to look authentic to risking violating anti-discrimination laws.

Homeland Security Secretary Janet Napolitano has said investigations will focus on businesses that knowingly hire immigrants who cannot legally work in the U.S.