The New York City contractor that provides medical and mental health services to inmates at Rikers Island was issued the highest level of censure by the federal Labor Department on Thursday for failing to protect employees from violence at the jail complex.

A six-month investigation by the Occupational Safety and Health Administration, which is overseen by the department, found that the contractor, Corizon Health Inc., “failed to address the serious problem of assaults against its employees,” Robert Kulick, OSHA’s regional administrator in New York, said in a statement.

The agency fined Corizon $71,000 and issued a “willful violation” against the company, because it was aware of the problem and failed to do anything about it, said Andre J. Bowser, a spokesman for the agency. He said it was only the second time in the last three years that the agency had issued such a serious citation for workplace violence against an employer.

Attacks by inmates against Corizon employees, including doctors, nurses and mental health workers, jumped from eight in 2011 to 39 in 2013, the investigation found. And six assaults on employees occurred between Feb. 11 and May 14 this year. An article published this year in The New York Times highlighted the surge in attacks.