The Federal Aviation Administration said on Wednesday that is has found an issue with the Boeing 737 Max that the manufacturer must address before it lifts the national grounding order.

“The FAA is following a thorough process, not a prescribed timeline, for returning the Boeing 737 Max to passenger service. The FAA will lift the aircraft’s prohibition order when we deem it is safe to do so,” the agency said in a statement. “The FAA’s process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing must mitigate.”

The issue was discovered during a simulator test last week, Reuters reported. The 737 Max has been grounded since March after two deadly crashes involving the plane. Regulators around the world have pointed to a software issue as a potential cause of the accidents.

A source close to the investigation told NBC News that FAA pilots found an issue during a simulation last week of the Maneuvering Characteristics Augmentation System (MCAS). The system took too long before allowing the pilots to recover control of the plane. 

Boeing has traced the issue to a microprocessor and how the chip handles data, NBC News reported. The company believes it can address the issue with a software code update.

“The safety of our airplanes is Boeing’s highest priority. We are working closely with the FAA to safely return the Max to service,” a company spokesperson said in a statement to CNBC.

Shares of the aerospace company dropped more than 1% following the news, but closed the day up.

—Reuters and NBC News contributed to this report.