WASHINGTON (Reuters) - The chair of a panel of international regulators that harshly criticized the Federal Aviation Administration’s certification of the now grounded Boeing 737 MAX will testify Tuesday before a Senate committee.

FILE PHOTO: National Transportation Safety Board Chairman Christopher Hart testifies before a House Transportation and Infrastructure hearing on the recent deadly Amtrak accident in Philadelphia, on Capitol Hill in Washington June 2, 2015. REUTERS/Jonathan Ernst

Christopher Hart, a former National Transportation Safety Board chair who oversaw the review, will speak before the Senate Commerce Committee alongside current NTSB chairman Robert Sumwalt, the panel said in a statement Wednesday.

They will appear after the testimony of Boeing Co BA.N chief executive Dennis Muilenburg and John Hamilton, who is vice president and chief engineer for Boeing Commercial Airplanes. Muilenburg will then testify before a U.S. House panel on Wednesday.

The testimony will take place on the first anniversary of the crash of a Lion Air 737 MAX jet that killed all 189 on board. On Wednesday, Indonesian investigators told victims’ families that mechanical and design issues contributed to the crash.

Contributing factors to the crash included incorrect assumptions on how an anti-stall device, called the Maneuvering Characteristics Augmentation System (MCAS), functioned and how pilots would react, slides in the presentation showed.

Congress is mulling changes to how the FAA delegates some certification tasks to manufacturers for new airplanes. In the 737 MAX certification it initially delegated 40% of the work to Boeing and later shifted more work to Boeing, including MCAS.

The report from the Joint Authorities Technical Review (JATR), commissioned by the FAA, faulted the review of MCAS and Boeing for assumptions it made in designing the airplane.

On Friday, Boeing turned over to the FAA instant messages from November 2016 from a former Boeing technical pilot to another pilot that it had withheld for months, raising questions about what the company knew about MCAS. Boeing has suggested the messages raised concerns about the simulator and not MCAS itself. FAA demanded an immediate explanation for why the messages were not turned over earlier.

The JATR recommendations said the FAA’s longstanding practice of delegating “a high level” of certification tasks to manufacturers needs significant reform to ensure adequate oversight. The FAA failed to “to provide an independent assessment of the adequacy” of Boeing’s MCAS certification activities.

On Tuesday, FAA Administrator Steve Dickson said the agency had received the “final software load” and “complete system description” from Boeing on the 737 MAX, but added it is going to be several more weeks before a key certification test flight. Major U.S. airlines have canceled 737 MAX flights into January and February.