By Dana Kozlov

CHICAGO (CBS) — After two catastrophic crashes involving the 737 Max 8 that left almost 400 dead, there are serious new questions about how it was approved to fly.

Congress plans to investigate the approval process of Boeing’s 737 Max 8.

That information comes on the heels of a Seattle Times report that the FAA may have sidestepped its oversight duties, allowing Boeing to essentially certify the aircraft itself.

“The flight data shows that they were very similar characteristics, that the plane went nose down at a not too far different altitude,” said Kevin Durkin, the aviation expert for Clifford Law Offices. “I believe it’s going to turn out that they were identical because of a single sensor, which caused the plane to nose dive.

He believed that potential flaw in the 737 Max 8’s design is part of an investigation. American Airlines Captain Dennis Tajer said lack of training to handle it should also be investigated.

“That aircraft was certified without simulator training,” Tajer said.

Now the spotlight is on how it was certified in the first place.

A Seattle Times report states that the FAA may have handed off some or all of the approval process to Boeing itself.

“It’s a deep concern,” Tajer said.

“The FAA should oversee everything,” Durkin said. “They are the ultimate authority in the United States to say these aircraft are safe or not safe, and they should not leave it to the manufacturer to make that decision alone.”

An FAA representative told CBS 2, “The 737 Max certification program followed the FAA’s standard certification process. We have no reports from whistleblowers [or] any other sources pertaining to FAA technical personnel being pressured to speed up certification of the Boeing 737 Max.”

“As the facts from the accident become available and we understand the necessary next steps, we’re taking action to fully reassure airlines and their passengers of the safety of the 737 Max,” said Boeing CEO Dennis Muilenburg.

“Our pilots are now getting a chance to reset and see all the facts, and I can assure you that aircraft’s not going to fly again until the pilot’s say, ‘I’m confident in it. I’m trained, and I’m knowledgeable,” Tajer said.

Durkin said it has become more common over the past 20 years for the FAA to hand over some of its oversight to manufacturers because the FAA is strapped for resources.