STORY: Boeing chief executive Dave Calhoun endured a tough grilling by U.S. lawmakers on Tuesday.

He appeared before a Senate committee to be questioned about safety lapses at the jet maker.

Calhoun began by addressing the two fatal crashes of 737 MAX planes that killed 346 people some years ago:

"I would like to apologize on behalf of all of our Boeing associates spread throughout the world, past and present for your losses. There are gut wrenching and I apologize for the grief that we have caused."

Lawmakers went on to question him over the midair blowout on an Alaska Airlines MAX jet earlier this year - an incident that raised new questions over the firm's quality control.

Calhoun said that was down to an isolated manufacturing defect, not a design flaw as in the fatal crashes.

"Alaska, very different. That was a manufacturing miss."

Boeing has since delivered a quality improvement plan to regulators.

But it has also faced accusations that it sought to suppress whistleblowers who raised new concerns.

Calhoun said the firm was working hard to listen to complaints and address any safety issues raised.

His answers didn't satisfy the committee chair, Democratic Senator Richard Blumenthal:

"The manufacturing issues, the retaliation against whistleblowers, nonconforming parts, quality inspections skipped and issues concealed from the FAA, evidence hidden, all have continued... In fact, there is near overwhelming evidence, in my view, as a former prosecutor, that prosecution should be pursued."

The U.S. Justice Department found in May that Boeing had failed to honor the terms of a deferred prosecution agreement that followed the fatal crashes.

Prosecutors have until early July to inform a federal judge of their next move.

Calhoun has pledged to step down as CEO by the end of the year, but was pressed by some lawmakers over why he didn't just go now.