ATLANTA (AP)-- Federal immigration authorities are beginning a new round of investigations designed to make sure businesses hire only people authorized to work in the U.S., focusing this time on companies vital to national security and other government and economic functions.

U.S. Immigration and Customs Enforcement said Wednesday it is notifying 1,000 companies that it will inspect their I-9s, the forms new employees complete, along with documents the workers provide to show they are eligible to work in the U.S.

"The inspections will touch on employers of all sizes and in every state in the nation, with an emphasis on businesses related to critical infrastructure and key resources," ICE said in a statement.

ICE declined to name the businesses or give their locations, citing the sensitive nature of the inspections.

The Obama administration has made cracking down on employers a key part of its immigration enforcement policy, emphasizing employer audits more than the high-profile workplace raids done during the Bush administration.

This is the latest in a series of employer investigations by federal immigration authorities. The most recent previous audit was in February, when ICE investigated 1,000 employers. This new action brings the fiscal year 2011 I-9 audit total to 2,338 audits, up from the previous fiscal year.