With the 737 Max still grounded and the world waiting for investigators to explain last week's deadly Ethiopian Airlines crash, the focus is increasingly on Boeing. Sources tell CBS News the fraud unit at the Department of Justice subpoenaed documents, records and data relating to the 737 Max approval process.

"These kinds of cases hinge on what did Boeing know, when did they know it, and who did they tell about it -- or not tell about it," said David Gomez, a retired FBI executive with experience in crash investigations.

A safety assessment Boeing gave to the Federal Aviation Administration (FAA) is likely to be a target of investigators. The Seattle Times reported the assessment appeared to downplay the operation of the anti-stall system believed to have repeatedly pushed the nose down on a Lion Air 737 Max that crashed in October.

"If there is any kind of documentation that indicates that they knew there was a problem and either didn't resolve 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.

Boeing has been making test flights to evaluate the software updates they plan to install to fix the Max. The company will no longer charge airlines for a warning light that would alert pilots to a sensor malfunction, like the one believed to have set off the Lion Air accident.

Previously, it was a pricey add-on feature. But it appears those Ethiopian and Lion Air pilots didn't have the warning because their airlines would have had to pay for it, on a plane that has a list price of about $120 million.

The Defense Department called for a review of its Boeing aircraft including Air Force One as a precaution. Ethiopian investigators are pledging to speed up their inquiry and release a preliminary report in eight to 10 days.