Both the US Department of Justice and the Federal Aviation Administration are examining various safety problems on the 737 MAX assembly line, The Wall Street Journal reported Tuesday. The report follows Boeing’s February admission that it found tools, rags and other debris in the fuel tanks of more than 30 737 MAX planes.

The appearance of debris can stem from “quality-control lapses” like those that have been flagged for prosecutors, according to the Journal.

The criminal and civil inquiries reportedly build on a grand jury probe of the plane’s problematic flight control systems. That investigation has focused on what Boeing staff told the FAA about dangers with the plane before it was involved in two crashes that killed 346 people.

Both the Justice Department and the FAA have interviewed Ed Pierson, a former Boeing manager who raised red flags to the company and the feds about safety problems at Boeing’s factory in Renton, Washington, according to the paper.

Pierson has also told the House Transportation Committee about how Boeing’s push to speed up production allegedly led to “chaos” at the factory. The committee and the Justice Department were “the only two entities which responded with any sense of alacrity or urgency,” Pierson’s attorney, Eric Havian, told the Journal.

Boeing declined to comment on the reported investigations Tuesday. But the company said it launched an internal investigation and took corrective actions after finding debris in undelivered 737 MAX planes.

“Safely returning the 737 MAX to service is our top priority,” Boeing said in a statement.

The 737 MAX — which was once Boeing’s fastest-selling airplane — has been grounded for more than a year since the pair of fatal crashes in Indonesia and Ethiopia. The fallout from the crashes led the Chicago-based company to oust CEO Dennis Muilenburg and replace him with David Calhoun.