WASHINGTON (Reuters) - The U.S. Federal Bureau of Investigation has told passengers who were on an Alaska Airlines Boeing 737 MAX 9 that suffered a Jan. 5 mid-air emergency that they may be victims of a crime, according to letters seen by Reuters.

The letters, a procedural step in some criminal investigations by the Justice Department, are a sign that its probe into the MAX 9 emergency is moving forward.

The letters, dated Tuesday, say the FBI has identified the passengers "as a possible victim of a crime. This case is currently under investigation by the FBI. ...A criminal investigation can be a lengthy undertaking, and, for several reasons, we cannot tell you about its progress at this time."

The letters were earlier reported by the Seattle Times.

An FBI spokesperson in Seattle declined to comment, citing Justice Department policy that it "does not confirm or deny the existence of an investigation."

The Justice Department in 2022 updated its guidelines for notifying victims of potential crimes after relatives of some of the 346 people killed in two Boeing 737 MAX 8 crashes in 2018 and 2019 said their legal rights had been violated when the department struck a January 2021 deferred prosecution deal with Boeing ( BA ) without notifying them.

The department also apologized for not meeting with relatives of the crash victims before the announcement of the deal which capped a 21-month investigation into the design and development of the 737 MAX 8.

Boeing ( BA ) declined to comment on Friday but said this month said it would continue to cooperate fully and transparently with all government investigations.

Alaska Airlines said it was fully cooperating with the investigation and does not believe it is the target of the probe.

In the midair emergency, a door plug panel tore off from the side of the MAX 9 jet at 16,000 feet, leaving a refrigerator-sized, rectangular hole in the aircraft. The plane landed safely with all 171 passengers and six crew on board. Seven passengers and one flight attendant had minor injuries.

The National Transportation Safety Board has said four key bolts appeared to be missing from the plane that had been delivered by Boeing ( BA ) months earlier. Boeing ( BA ) has said it believes required documents detailing the removal of the bolts were never created.

In the aftermath of the incident, the FAA grounded the MAX 9 for several weeks, barred Boeing ( BA ) from increasing the MAX production rate and ordered it to develop a comprehensive plan to address "systemic quality-control issues" within 90 days.