Boeing’s path to returning its 737 Max jet to service is being held up by questions from the Federal Aviation Administration.

The U.S. regulator has asked Boeing to provide more details about “how pilots interact with the airplane controls and displays in different flight scenarios,” the company said in a statement Thursday. A spokesman for the FAA confirmed the agency is asking about pilot interaction on the 737 Max but declined to provide more information about the review process or how long it is expected to take.

“This process will take as long as necessary to ensure the aircraft is returned to service safely,” FAA spokesman Gregory Martin said.

More than two months after a deadly plane crash led to the worldwide grounding of the 737 Max, Boeing is facing mounting pressure from investors, airline customers and suppliers to get its flagship jet back into the air. Airlines are incurring millions of dollars in costs as planes sit idle and Boeing continues to produce 42 planes a month, which it cannot sell until the jet has been cleared by regulators.

Boeing’s timeline for a fix has been a moving target. The company has yet to submit its final package of software fixes to the FAA for approval, something it said it would do by early April. CEO Dennis Muilenburg said in late April that the company had completed its final technical test flight with the updated flight control system and was preparing for a certification flight and formal review by the FAA.

In its statement Thursday, Boeing repeated its claim that its update for the 737 Max software has been completed. Once it has addressed the FAA’s questions, “Boeing will work with the FAA to schedule its certification test flight and submit final certification documentation,” the company said. Boeing said the flight controls have been tested on 360 hours of flights, up from 246 hours in late April.

The FAA’s acting administrator, Daniel K. Elwell told members of Congress this week that the decisions of pilots played a role in the crashes of 737 Max jets in Indonesia and Ethiopia. An automated anti-stall feature activated when it wasn’t supposed to in the October crash in Indonesia, Elwell said. That sent the plane’s nose down repeatedly. The pilots should have responded by turning off the motors to the part of the airplane that was forcing it downward, but they did not, Elwell said.

When the same automated feature — known as the Maneuvering Characteristics Augmentation System, mistakenly kicked in less than five months later on the 737 Max involved in the Ethiopia crash — the pilots “didn’t adhere to the emergency” directive issued by the FAA in November, Elwell said.