The pilots union at Southwest Airlines Co., the largest operator of the 737 Max, wants to be repaid by Boeing Co. for lost pay and other costs related to the aircraft’s grounding after two fatal crashes.

There’s no estimate on when the model, parked worldwide on March 13, will be allowed to resume flights, as Boeing continues testing new software to fix a flight system linked to the accidents that killed a combined 346 people. In addition, the manufacturer and U.S. safety regulators still must suggest new pilot training and emergency procedures.

The Southwest Airlines Pilots Association is the first labor group to say it will join Max operators in seeking to recover costs from Boeing, increasing the planemaker’s potential financial obligations from the accidents. The union says pilots have lost at least $9 million a month in pay while the airline’s 34 Max aircraft have been grounded, and Swapa has spent money to comply with a U.S. Justice Department request for records and documents related to the plane.

“This and other Swapa expenses relating to the Max, as well as the loss of flying to our members, is very expensive,” Jon Weaks, the union’s president, said in a message to members Wednesday. “Swapa will be seeking compensation and reimbursement from Boeing for every dollar legally available to be challenged when the Max issues are resolved.”

Southwest declined to comment. Boeing didn’t respond to requests for comment.

CNBC earlier reported the union’s plan to recover Max-related costs from Chicago-based Boeing.

Southwest last week pulled the Max from its flight schedule for an additional month, through Sept. 2, citing uncertainty over when regulators will allow the jet to resume service. The Dallas-based carrier is canceling about 100 daily flights as a result.