Lawyers representing families of passengers killed in a Boeing 737 Max crash in Ethiopia in March are set to issue subpoenas to Southwest Airlines and American Airlines, the two biggest US operators of the jet, according to documents seen by Reuters.

The subpoenas will be issued over the next couple of days, the lawyers separately told Reuters.

The lawyers want to know what Boeing 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 29 October 2018, and before the Ethiopian Airlines crash on 10 March.

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.

Southwest and American did not immediately 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.

In a court filing late Monday, Boeing said veteran litigator Dan Webb, a high-profile Chicago lawyer and former US attorney, had joined its legal team in the Lion Air case.

In addition to the Chicago cases, questions about how Boeing designed and developed the 737 Max are a focus in investigations by the US Department of Justice and a congressional committee.

The plane-maker has said it was sorry for the lives lost in both crashes but stopped short of admitting liability in how it developed the 737 Max or the flight control software.

Last week, an international aviation panel criticized US regulators and Boeing over the certification of the plane.