New York (CNN) US Justice Department prosecutors have issued multiple subpoenas as part of an investigation into Boeing's Federal Aviation Administration certification and marketing of 737 Max planes, sources briefed on the matter told CNN.

The criminal investigation, which is in its early stages, began after the October 2018 crash of a 737 Max aircraft operated by Lion Air in Indonesia, the sources said. Transportation Secretary Elaine Chao on Tuesday asked the agency's inspector general to investigate the Max certification.

Criminal investigators have sought information from Boeing on safety and certification procedures, including training manuals for pilots, along with how the company marketed the new aircraft, the sources said.

It's not yet clear what possible criminal laws could be at issue in the probe. Among the things the investigators are looking into is the process by which Boeing itself certified the plane as safe, and the data it presented the FAA about that self-certification, the sources said.

The FBI Seattle office and Justice Department's criminal division in Washington are leading the investigation.

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