The US Transportation Department said it had been hit by a data breach involving its administrative systems.

A preliminary inquiry found that the breach was confined to software used to process employee transit benefits, the department said in a brief statement on Friday night. No transportation safety systems were affected, according to the statement.

Since detecting the intrusion the department has enlisted the help of the Cybersecurity and Infrastructure Security Agency and has “suspended access to relevant systems while we further investigate the issue, and secure and restore the systems,” the department said in the statement, which did not elaborate further.

According to Reuters, which reported the breach earlier, as many as 237,000 current and former workers are believed to have been affected.

The department as well as its boss, Transportation Secretary Pete Buttigieg, came under criticism early this year after flights across the US were grounded because of a computer system failure at the Federal Aviation Administration.