In an open letter addressed to airlines, passengers and the aviation community, Dennis Muilenburg says Boeing will soon release a software update and offer related pilot training for the 737 Max to "address concerns" that arose in the aftermath of October's Lion Air flight that plunged into the Java Sea, killing 189. The planes' new flight-control software is suspected of playing a role in the crashes.

Muilenburg says Boeing representatives are supporting the investigation into the cause of last week's crash of an Ethiopian Airlines Max 8 that killed 157.

The United States and many other countries have grounded the Max 8s and larger Max 9s as Boeing faces the challenge of proving the jets are safe to fly amid suspicions that faulty sensors and software contributed to the two crashes in less than five months.

U.S. prosecutors are looking into the development of Boeing's 737 Max jets, a person briefed on the matter revealed Monday, the same day French aviation investigators concluded there were "clear similarities" in the crash of an Ethiopian Airlines Max 8 last week and a Lion Air jet in October.

The Justice Department probe will examine the way Boeing was regulated by the Federal Aviation Administration, said the person, who asked not to be identified because the inquiry is not public.

A federal grand jury in Washington sent a subpoena to someone involved in the plane's development seeking emails, messages and other communications, the person told The Associated Press.