"At this moment an entire fleet of U.S. made aircraft is grounded due to two tragic accidents overseas," Dickson said. "My heart, my prayers, go out to the families of those who perished in Indonesia and Ethiopia."
Acting FAA Deputy Administrator Carl Burleson responded to those reports in a Senate subcommittee hearing on July 31 that discussed the FAA's oversight of the Boeing 737 MAX.
"The professionals who are working this day in and day out have an incredible commitment to trying to get it right," Burleson said at the hearing. "The fundamental process of how we went about certifying the MAX was sound."
Dickson said on Monday that the FAA is not following any timeline for returning the aircraft to service.
"We're going where the facts lead us and diligently ensuring that all technology and training is present and current before the plane returns to passenger service," Dickson said.
In June, FAA pilots found a new potential issue with the 737 Max aircraft involved in both fatal crashes during a simulated flight, according to two sources familiar with the matter. The new flaw was traced to how data was processed by the flight computer and not related to reported problems with the anti-stall system, MCAS, sources told ABC News. They said it was connected to a broader anti-stall system called "speed trim."
Last week, Boeing CEO Dennis Muilenburg said the company had worked its way through the "technical details" and are "in the final stages of repairing that software."
"We'll go through certification with the FAA," Muilenburg said. "We plan to submit that certification package in September and currently anticipate that we will return the airplane to service early in the fourth quarter."
ABC News' Soo Youn contributed to this report.