A unit of Smithfield Foods Inc. (SFD) paid a $290,400 penalty over allegedly discriminatory hiring practices at one of the its Illinois slaughterhouses, the Department of Justice said Monday.

The agency's Civil Rights Division in June sued Smithfield's Farmland Foods for requiring workers at its Monmouth, Ill., pork-processing facility to present more work-authorization documents than required by federal law.

The penalty comes amid a simultaneous federal crackdown on illegal U.S. workers that has added pressure on employers to verify their own employees' legal status. In Farmland's case, the company allegedly went too far by discriminating against U.S. citizens or foreigners with valid work authorization.

Farmland allegedly required non-U.S. citizens to provide documents such as social security cards to work even when employees had other documents establishing their legal status. The company also demanded evidence of naturalized U.S. citizen's status beyond the documents required by law, the Justice Department said.

Requiring specific or excessive documents to establish work authority violates the antidiscrimination provision of the Immigration and Nationality Act. The Justice Department called the latest fine against Farmland the largest civil penalty reached through settlement since the provision's enactment in 1986.

Farmland also agreed to monitor and report certain hiring practices and training its human resources personnel, the department said. A representative of Smithfield Foods declined to immediately comment.

"The Justice Department is committed to protecting the right of all work-authorized employees, regardless of their citizenship or immigration status, to work without having to overcome extra and discriminatory hurdles during the hiring process," assistant attorney general Thomas E. Perez said.

Smithfield shares were recently up 3 cents at $19.43. The stock has gained 27% over the past year. Read more about Smithfield's profit outlook.