Boeing 737 Max jets remain grounded in the U.S. and around the world following the deadly Lion Air and Ethiopian Airlines crashes that occurred within just months of each other and killed 346 people.

The FBI has joined a growing list of those demanding answers which now includes the Transportation Department, members of Congress and the Pentagon's inspector general office, which has launched an ethics investigation into Acting Defense Secretary Patrick Shanahan, a former Boeing executive, to see if he inappropriately pushed for Boeing products. Boeing is the nation's largest exporter and one of the world's biggest defense contractors.

Sen. Richard Blumenthal asked Shanahan if he supports such an investigation last week. He replied, "Yes I do."

In a statement, Shanahan said he is committed to upholding his ethics agreement with the Defense Department. Boeing executives are expected to be called to testify about the 737 Max at a Senate hearing next week.

Indonesian investigators confirmed an off-duty pilot was in the cockpit of the same Lion airplane that experienced trouble, one day before a deadly crash. They did not say whether he helped stabilize the plane. The lead investigator said the off-duty pilot was not in their initial report on the October crash because investigators had not spoken to him yet.

Boeing 737 Max family of planes are shown in a photo provided by the company. Boeing

After last week's crash of Ethiopian Airlines Flight 302, a Boeing employee received a subpoena to retain records relating to the approval of the 737 Max. Department of Transportation inspector general agents told employees at the FAA's Seattle office to do the same.

"What did the FAA and Boeing know? When did they know it? And did they tell anybody about it or did they hide that?" said David Gomez, a retired FBI executive with experience in crash investigations.

The probe is focused on the process to certify the 737 Max as safe and airworthy, a process that by design allows the manufacturer to certify much of the plane itself.

"If there is any kind of documentation that indicates that they knew there was a problem and either didn't resolve it to the satisfaction of the FAA or didn't reveal that, that could put them in jeopardy in terms of a possible criminal violation," Gomez said.