Lawmakers interrogated the acting head of the Federal Aviation Administration , the chairman of the National Transportation Safety Board and the inspector general from the Department of Transportation about the certification and oversight of the 737 Max program at two hearings in late March. The House Aviation Subcommittee is hauling officials from the FAA and NTSB back to Capitol Hill for another hearing next week.

In two hearings held in late March and one scheduled for next week, U.S. lawmakers are grilling several officials involved in the 737 Max investigation in the U.S. — except Boeing CEO Dennis Muilenburg .

Nearly two months since aviation regulators around the world grounded Boeing's 737 Max airplanes, there are still plenty of questions swirling around Washington, D.C., about the plane's development, certification and overall safety.

So why hasn't Congress called Muilenburg — or any other company executive, for that matter — to testify on Capitol Hill about the two crashes in recent months in Indonesia and Ethiopia that killed a combined 346 people.

"Are they truly interested in total transparency? Do they want to hear from the top people?" asked former Transportation Secretary Ray LaHood. "I do think there is a little bit of doubt for the moment here why the CEO has not testified."

Sen. Roger Wicker, R-Miss., who oversees the Senate Commerce, Science and Transportation Committee intends to call Boeing once the 737 Max is recertified, according to a spokesperson. The House Committee on Transportation and Infrastructure is just beginning its hearings, according to a spokesperson for Chairman Peter DeFazio, adding that there are no plans at this point to hear from Muilenburg.

Boeing spokeman Chaz Bickers says the company "continues to work with Congress on information requests from members." Congress hasn't called any Boeing executives to testify on the 737 Max yet, he confirmed.

But Congress has been relatively quiet when it comes to the 737 Max, despite high-profile grillings of CEOs at other companies caught in the midst of a public safety scandal.

"It's a puzzle in a lot of ways," said Norm Ornstein, of the American Enterprise Institute. Ornstein has spent years studying Congress and has seen this act before. "We now have ample evidence of Boeing backtracking and double-talking about what went wrong with the Max, so there's enough to lead Congress to question the CEO."

In the past, Congress has moved relatively quickly to hold CEOs accountable when lives have been at risk.

In 2017, United Airlines CEO Oscar Munoz was called to Capitol Hill to explain his company's treatment of passengers just three weeks after Dr. David Dao was dragged off of a United plane preparing to take off.

In 2014, General Motors CEO Mary Barra, was grilled by senators about the safety of GM vehicles less than two months after millions of GM cars and SUVs were recalled for faulty ignition switches.

In 2010, Toyota CEO Akio Toyoda went before lawmakers to defend the safety of Toyota vehicles a month after a major recall involving models that may have faulty accelerator pedals.

Ornstein said the Russia investigation may be distracting lawmakers from focusing on the 737 Max crisis.

Few Americans were impacted by the crashes, lessening the usual public outrage, he said.

"The fact is, we are dealing with two tragedies that happened far from home and for now the planes are not flying. So there's less pressure on Congress to get answers," he said. "If these accidents were in the U.S., I think we would probably have seen a stronger reaction in the Senate and House by now."

Boeing also has deep ties with power brokers on Capitol Hill and spends heavily to influence policy.