The uncertainty over the cause of the latest crash has put Boeing on the defensive. The 737 Max is the company’s best-selling jet ever, and it is expected to be a major driver of profits in the future. Around 5,000 of the planes are on order, with a list price of $120 million for the Max 8 version.

Shares of the company fell 5 percent on Monday, as pressure mounted from various corners.

The Association of Flight Attendants sent a letter to the F.A.A. on Monday calling for a review of the 737 Max. “We need help from the regulators when the entire world is looking at two catastrophic incidents that happened on the same aircraft type within five months of each other,” said Sara Nelson, the president of the flight attendants’ union. “Our system is so safe that these things don’t happen today. That is why people are questioning what is going on here.”

Pilots also raised questions about the safety of the plane. “We’re very concerned about why two brand-new aircraft suddenly pitched over and nosed into the ground,” said Rory Kay, a former top safety official at the world’s largest pilot union and a senior pilot and pilot trainer at a major United States airline. “This is not the dawn of aviation. We’ve evolved, planes have evolved.”

Dennis Tajer, a spokesman for the American Airlines pilot union and a 737 pilot, said that the Lion Air crash hurt the reputation of the 737 Max in the eyes of some of his members and that the Ethiopian accident had prompted new questions from them. “I think there needs to be further review into the certification process” for the aircraft, Mr. Tajer said. “Everybody should be looking at this.”

In Washington, two senators, Dianne Feinstein of California and Richard Blumenthal of Connecticut, went a step further, calling on the F.A.A. to ground all Boeing 737 Max 8s until the investigation into the Ethiopia crash was complete.

Some passengers tried to rebook their flights to avoid a 737 Max 8. Carriers that hadn’t grounded the plane sought to reassure customers, providing information about the jet’s safety.