https://phys.org/news/2019-03-probing-certification-boeing-max.html

The subpoena, which came from a prosecutor in the Justice Department's criminal division, seeks documents and correspondence related to the plane, according to the report.

A criminal inquiry is "an entirely new twist," said Scott Hamilton, managing director of the Leeham Company, who recalled a probe of a 1996 ValuJet crash as the only other aviation probe that was not a civil investigation.