CHICAGO (Reuters) - Unions representing flight attendants at the three U.S. airlines that operate Boeing Co's BA.N 737 MAX said they are more reluctant to support the grounded jet's return after congressional hearings this week on its development.

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Southwest Airlines LUV.N and United Airlines UAL.O flight attendant union leaders joined colleagues' demands for an active role in regulators' and airlines' decisions to let the 737 MAX fly again after two fatal crashes in Indonesia and Ethiopia.

The two unions spoke after news that the head of American Airlines' AAL.O flight attendants union had sent Boeing Chief Executive Dennis Muilenburg a letter saying that the Washington hearings raised questions about 737 MAX supervision and oversight.

“The emails, the testimony... it all leads to more questions than answers,” Chad Kleibscheidel of the Southwest flight attendants union said in reference to employee memos about the aircraft’s safety before it was certified in 2017 that came to light during back-to-back hearings.

Southwest is the world’s largest 737 MAX operator and has over 15,000 flight attendants trained to fly the aircraft.

Sara Nelson, president of the Association of Flight Attendants-CWA, which represents flight attendants at 20 airlines including United, said: “This week took a step backward in this process, not forward.”

Boeing spokesman Gordon Johndroe said in an emailed statement that the company is committed to providing flight attendants, pilots and its airline customers the information they need to re-earn their trust and that of the traveling public.

Southwest and United executives have told Reuters that flight attendants - along with pilots, call center agents and gate agents - will play a key role in defending the jet to passengers once it is approved to fly again.

U.S. flight attendants were among the groups that called for the Federal Aviation Administration to ground the 737 MAX after an Ethiopian Airlines jet nose-dived after taking off from Addis Ababa on March 10, five months after a similar crash on a Lion Air flight in Indonesia. The two crashes killed 346 people.

Southwest, United and American are scheduling without the 737 MAX into early next year as the plane approaches a nine-month grounding, causing thousands of canceled flights for each airline and a hit to profit.

Southwest pilots have sued Boeing for lost wages during the grounding.

“Flight attendants have taken a big hit to their salaries too,” said Kleibscheidel, who is first vice president of the Southwest flight attendants union, Transport Workers Local 556 .

“People have bills to pay and mouths to feed. No planes, no pay.”