Dozens of U.S. Navy service members became ill after being exposed to diesel exhaust fumes from the Ohio-class ballistic missile submarine USS Nebraska on June 22.

Axar.az reports, citing Navy Times, sixty-four sailors experienced coughing, headaches, lightheadedness, dizziness and nausea while working a routine pier shift at Naval Base Kitsap-Bangor in Silverdale, Washington.

The service members were near a functioning backup diesel generator when the poisoning occurred. They also experienced irritation of the eyes, nose and throat.

“The Navy is working to identify the precise cause of the event and will take corrective action as appropriate,” the service said.

Six sailors were admitted for further medical attention, but all have since been released.