Boeing Co. (BA - Get Report) shares ended lower Thursday after the U.S. Federal Aviation Administration said it found yet another issue with the planemaker's 737 MAX software that could further delay its return to service.

The FAA said it found a "potential risk" in Boeing's MCAS software, reportedly during simulator tests last week, that will require additional fixes from the planemaker and delay its official testing date until at least July 8, according to a Reuters report. Bloomberg reported later in the session that the fix may take until at least September, noting that U.S. government test pilots discovered the glitch as the system became overloaded with flight data.

The FAA also said the testing and procedures were following a process, not a prescribed timeline, for the 737 MAX's recertification, while the International Air Transport Association called for additional training requirements for crews flying the 737 MAX. 

"The safety of our airplanes is Boeing's highest priority. During the FAA's review of the 737 MAX software update and recent simulator sessions, the FAA identified an additional requirement that it has asked the company to address through the software changes that the company has been developing for the past eight months," Boeing said in a statement published late Wednesday. "The FAA review and process for returning the 737 MAX to passenger service are designed to result in a thorough and comprehensive assessment. Boeing agrees with the FAA's decision and request, and is working on the required software."

Boeing shares closed 3% lower on the Thursday session at $364.02 each, a move that would mark a 14% decline for the stock since the fatal Ethiopian Airlines crash of a 737 MAX 8 jet on March 10.

U.S. carrier United Airlines (UAL - Get Report) , meanwhile, said Wednesday that it will pull all MAX aircraft from its summer schedule, resulting in around 3,200 cancellations over July and August, and doesn't expect Boeing's flagship plane to return until at least September 3, echoing a similar assessment earlier this month from Southwest Airlines (LUV - Get Report) .

"The FAA 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," the Administration said in a statement published on its website. "We also are responding to recommendations received from the Technical Advisory Board (TAB). The TAB is an independent review panel we have asked to review our work regarding 737 Max return to service."

Boeing CEO Dennis Muilenburg has admitted the planemaker will need time to win back confidence in the 737 MAX's airworthiness, but also noted that even with "steady progress" being made towards its re-certification, he would not put a firm date on its return, saying only it would happen "before the end of the year."

"It is important for us to focus on safety, we will get back up in the air when it is safe, that's the most important thing here," Muilenburg said. "We are very confident in the MAX family and the heart of the market where it is located."

Last month, International Air Transport Association director Alexandre de Juniac cautioned that the planemaker's grounded 737 MAX may not return to full service before the end of the summer, while Muilenburg told an investor conference in New York hat the company was focused on "safely returning the MAX to flight" and stabilizing its production rate at 42 units per month.

The planemaker said the preliminary report into the cause of Ethiopian Airlines Flight 302's fatal crash on March 10, as well as the Lion Air 610 disaster in Indonesia in early October, which took the lives of 189 people, were caused by activation of the Maneuvering Characteristics Augmentation System, or MCAS, in response to "erroneous angle of attack information" from a broken sensor.