By Agence France-Presse

US regulators said Wednesday Boeing must address a new “potential risk” in the Boeing 737 MAX, further clouding the timeframe for resuming service on the planes after two deadly crashes.

The issue, which surfaced during FAA simulator testing, concerns the ability of pilots to quickly reassert control of the plane if an automated flight handling system pushes the plane downward, said a person familiar with the matter.

The Federal Aviation Administration “will lift the aircraft’s prohibition order when we deem it is safe to do so,” the agency said in an email.

“The FAA recently found a potential risk that Boeing must mitigate.”

Boeing said the software fix for the 737 MAX that it has been developing for the last eight months does not currently address the matter.

“The Boeing Company agrees with the FAA’s decision and request and is working on the required software to address the FAA’s request,” Boeing said in a securities filing.

“Boeing will not offer the 737 MAX for certification by the FAA until we have satisfied all requirements for certification of the MAX and its safe return to service.”

Boeing’s global fleet of 737 MAX planes has been grounded since mid-March following two crashes which claimed 346 lives.