The U.S. Federal Aviation Administration (FAA) has identified a new potential risk that Boeing Co must address on its 737 MAX before the grounded jet can return to service, the agency told Reuters on Wednesday.

The risk was discovered during a simulator test last week, sources with knowledge of the matter told Reuters. As a result, Boeing is not expected to run a certification test flight until at least July 8, they said.

“On the most recent issue, the FAA’s process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing must mitigate,” the FAA said in the statement emailed to Reuters.

Asked about the new potential risk, Boeing said it is “working closely with the FAA to safely return the MAX to service.”