Lawyers representing families of passengers killed in a Boeing 737 MAX crash in Ethiopia in March are set to issue subpoenas or orders to Southwest Airlines and American Airlines, according to documents.

The summons will be issued to the two biggest U.S. operators of the jet over the next couple of days, the lawyers said.

The lawyers want to know what Boeing Co promised potential airline customers about flight crew training and the 737 MAX certification process, and its communications with the two airlines following a Lion Air crash in Indonesia on Oct. 29, 2018, and before the Ethiopian Airlines crash on March 10.

They are seeking documents about 737 MAX software known as MCAS widely linked to both deadly crashes, as well as information on the aircraft's sensors.

The two crashes, both involving 737 MAX jets, killed 346 people and spurred more than 100 lawsuits against the Chicago-based planemaker.

American declined to comment, while Southwest did not immediately respond to a request for comment.

The 737 MAX was grounded globally following the Ethiopian crash in March, and Boeing has been working to deliver software fixes aimed at winning fresh approval for commercial flight in the fourth quarter. Southwest and American have each canceled more than 100 daily flights.

While families of the Lion Air crash victims are in settlement talks with Boeing, the Ethiopian crash victims are pursuing a jury trial.

Both cases are in Chicago federal court, where a Lion Air status hearing is taking place on Thursday.