June 26 (UPI) -- The U.S. Federal Aviation Administration said Wednesday it has found a new "risk" with Boeing's best-selling 737 Max aircraft that must be addressed.
"The FAA recently found a potential risk that Boeing must mitigate," the federal administration said in a statement without elaborating.
Boeing said it agrees with the FAA's decision and is working to fix the problem.
"The FAA review and process for returning the 737 Max to passenger service are designed to result in a thorough and comprehensive assessment," Boeing said in a statement. "Boeing agrees with the FAA's decision and request, and is working on the required software."
Boeing said the FAA identified the risk while reviewing a software update during recent simulator sessions and has asked for further changes.
The family of 737 Max aircraft will not receive certification from the FAA until it has satisfied all requirements and is safe to return to service, the company said.
The announcement came over a month after Boeing completed a software update to the 737's Maneuvering Characteristics Augmentation System, which prevents a plane from stalling by sending it into a dive.
The embattled U.S. aircraft manufacturer has been working to re-certify the aircraft since it was grounded in March following the two fatal crashes.
Early this month, the FAA said that the plane may also contain "improperly manufactured parts," dealing another blow to Boeing.
The FAA said it is following a thorough process without a timeline for returning the Boeing 737 to service and "will lift the aircraft's prohibition order when we deem it is safe to do so."
"We continue to evaluate Boeing's software modification to the MCAS and we are still developing necessary training requirements," it said.