U.S. 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.

A Boeing (BA) spokesperson referred to a statement the company released earlier in the week, which indicated it “does not respond to or comment on questions concerning legal matters, whether internal, litigation, or governmental inquiries.”

The safety of the 737 Max has been called into question after it was involved in two fatal crashes in less than five months. Both the crash last October involving the Lion Air jet and the crash last week involving an Ethiopian Airlines jet resulted in the death of everyone aboard. (SD-Agencies)