Boeing CEO Dennis Muilenburg is testifying before a Senate panel on Tuesday, the anniversary of the Lion Air 737 Max crash that killed 189 people, in his first appearance before Congress since the two crashes.

Lawmakers are planning to ask Muilenburg about how the planes were designed, certified by regulators and marketed to airlines around the world as a fuel-efficient addition that didn't require time-consuming training for pilots.

The two crashes prompted a worldwide grounding of the 737 Max, Boeing's bestseller. They also spurred probes, including one by the Justice Department, of certification methods used by air safety regulators that handed over more tasks to the manufacturer, part of a decadeslong practice that lawmakers have permitted, drawing more scrutiny of government oversight of the industry.

Muilenburg took remorseful tone with lawmakers, according to prepared testimony released before the Senate Commerce Committee hearing. He also testifies Wednesday at the House Transportation and Infrastructure Committee.

"We know we made mistakes and got some things wrong," Muilenburg said. "We own that, and we are fixing them."The hearings come amid growing pressure on Muilenburg and other Boeing leaders. Boeing's board stripped Muilenburg of his chairman role earlier this month, saying separating the jobs would allow him to focus on getting the Max back to service as the timeline slipped. Last week, the company replaced the head of its key commercial airplane unit, which made the Max, after three years on the job.

In an interview with CNBC's Squawk Box on Tuesday, Sen. Ted Cruz, R-Texas, who chairs the Commerce Committee's aviation panel, said it's up to Boeing shareholders and its board to decide whether to fire Muilenburg. "I do think it's important that there be real accountability. There are a lot of questions that still need to be answered," he said.

Cruz said he wants to know why regulators signed off on the plane and why it lacked sufficient safeguards against malfunctions, and why were pilots not trained on the added system that was implicated in both crashes.

"How the hell did this happen?" Rep. Peter DeFazio, chairman of the House Transportation and Infrastructure Committee, before which Muilenburg is set to appear on Wednesday, told reporters on Monday. The Oregon Democrat said investors pressured Boeing into developing and marketing the plane quickly. "This all starts on Wall Street."

The crashes in Indonesia and Ethiopia killed a total of 346 people and prompted a worldwide grounding, which regulators still haven't lifted, dragging down profits at airlines that are hamstrung by the loss of the fuel-efficient planes from their fleets.

Muilenburg is appears with John Hamilton, a Boeing vice president and chief engineer of Boeing's commercial aircraft unit.

More than a dozen family members of victims plan to attend the hearings and meet with Muilenburg afterward. It will be the first time they have met with him, according to a spokesman for their attorney.