Boeing Co. (BA - Get Report) shares traded lower Thursday amid reports that lawmakers, federal investigators and the Department of Justice are lining up to question executives at the world's biggest planemaker over two horrific crashes of its 737 MAX planes that killed nearly 350 passengers and raised serious safety concerns for the flagship aircraft.

The Federal Aviation Administration is also under scrutiny for its role in approving Boeing's Maneuvering Characteristics Augmentation System, or MCAS, in March 2017. The software has been cited as the potential link between the October crash of a Lion Air flight in Indonesia and this month's Ethiopian Airlines disaster in east Africa.

Lawmakers on the Senate's subcommittee on aviation and space are set to question FAA officials on March 27, and said Wednesday they would invite Boeing executives to face similar queries later in the spring.

"In light of the recent tragedy in Ethiopia and the subsequent grounding of the Boeing 737 Max aircraft, this hearing will examine challenges to the state of commercial aviation safety, including any specific concerns highlighted by recent accidents," the panel, which is led by Republican Senator Ted Cruz, said Wednesday, adding it would like, "in the near future, to hear from industry stakeholders that would include Boeing, other aviation manufacturers, airline pilots, and other stakeholders."

Boeing shares were marked 0.33% lower Thursday at $375.00 each, a move that would extend the stock's slide since the fatal ET 302 crash, which killed all 157 passengers on board on March 10, to around 11.6%.

Bloomberg reported the FAA is also being probed by the Department of Justice, while CNN has said subpoenas have been issued in the DoJ's look into Boeing's culpability in the two crashes.

A further report, from the Seattle Times, suggests the Federal Bureau of Investigation is also looking into the MAX software certification process. A Wall Street Journal report earlier this week said the U.S. Department of Transportation is looking into the FAA's handing on the MCAS certification.

The FAA said last week that "new evidence" collected at the ET302 crash site compelled it to ground all "Boeing 737 MAX aircraft operated by U.S. airlines or in U.S. territory", reversing an earlier statement on the plane's safety.

"The grounding will remain in effect pending further investigation, including examination of information from the aircraft's flight data recorders and cockpit voice recorders," the agency added. "An FAA team is in Ethiopia assisting the NTSB as parties to the investigation of the Flight 302 accident. The agency will continue to investigate."

Beyond the horrific human cost of both the ET302 tragedy and the early October crash of a Lion Air flight out of Indonesia, which collectively took the lives of nearly 350 passengers, investors in Boeing, as well as companies linked to the 737's production in Renton, Washington, are looking into the impact on both its $600 billion order backlog and first quarter earnings.

Boeing's 737 lineup, perhaps its most important program, contributes around a third of the the Chicago-based planemaker's bottom line, as well as a fifth of its sales, and has a back order log of nearly 5,000 units.