Boeing was not able to respond to regulator questions on modifications made to the 737 MAX flight control system at an August meeting with international officials, sources said Tuesday.

The planes have been grounded globally since mid-March, following two crashes that killed 346 people. Boeing is now seeking a green light to return the jets to service after developing fixes for the jets’ flight control computers.

“Yes, we attended the meeting,” said a spokesperson for Brazil’s National Civil Aviation Agency, adding that Boeing was not able to answer regulators’ specific questions.

The meeting between Boeing and US, European and Brazilian regulators and others took place in Seattle, said another source, confirming prior reporting in The Wall Street Journal according to which the issue could push back Boeing’s timeframe for returning the 737 MAX to service.

The parties agreed to conclude the meeting and reconvene at a later date when Boeing could provide the details.

Boeing has said previously that it expects to submit its certification package to the FAA around September, with anticipated approval around a month later.

Questions about the flight control system surfaced during an FAA review in June separate from oversight of the Maneuvering Characteristics Augmentation System, an anti-stall system that has been seen as a factor in the two crashes.

A Boeing spokesman said the timetable for bringing the MAX back still holds.

“Our best current estimate continues to be a return to service of the MAX that begins early in the fourth quarter,” the Boeing spokesperson said.

“Our focus is on safety and ensuring the trust and confidence of customers, regulators and the flying public. Timing on return to service will be driven by the FAA and global regulators.”

A spokesperson for the regulator said “the FAA is following a thorough process, not a prescribed timeline, for returning the Boeing 737 MAX to passenger service.”

Shares of Boeing dropped 2.7 percent on Tuesday to $354.42.