The F.A.A. “put the fox in charge of the henhouse,” he said.

Boeing and the F.A.A. are playing defense on two fronts, as prosecutors, regulators and lawmakers investigate their responses to the crashes and their approval of the plane.

During his hearing, Calvin L. Scovel III, the Transportation Department’s inspector general, said he was looking into how the F.A.A. handled the crisis. In the days after an Ethiopian Airlines crash that killed 157 people this month, global regulators grounded the jets, while the United States initially held off doing the same.

“Clearly, confidence in F.A.A. as the gold standard for aviation safety has been shaken,” he said. Mr. Scovel said he wanted to understand why the agency was the last major aviation regulator in the world to “drive risk to zero” by grounding the 737 Max.

Mr. Scovel also said he would investigate why the F.A.A. approved the software system, which is known as MCAS, including the decision not to include it in the operating instructions and not to require additional training for pilots. When the plane was introduced, pilots learned about the 737 Max on an iPad.

Robert L. Sumwalt, the chairman of the National Transportation Safety Board, also said in the hearing that his agency was “examining the U.S. design certification process.”

The F.A.A. has long allowed plane makers to help certify that their new aircraft meet safety standards. In recent years, Boeing was able to choose its own employees to help regulators approve the 737 Max.

In a separate hearing with the Senate Appropriations Committee’s transportation subcommittee, the Transportation secretary, Elaine Chao, called the practice “necessary.”