The Department of Transportation’s inspector general is probing the Federal Aviation Administration’s approval of Boeing’s 737 Max 8 aircraft after new reports raised questions about the FAA’s reliance on the aircraft manufacturer during its certification process. Meanwhile, federal prosecutors are also looking into Boeing. The Wall Street Journal is reporting that a subpoena has been issued to at least one person involved in the Max 8’s development.

The DOT’s investigation is focused on the FAA’s Seattle office, which certifies the safety of new aircraft. The subpoena is seeking documents from the office, including emails, correspondence, and other messages, the Journal reports.

FAA managers pushed the agency’s safety engineers to delegate safety assessments to Boeing itself

Over the weekend, The Seattle Times reported that FAA managers pushed the agency’s safety engineers to delegate safety assessments to Boeing and to speedily approve the resulting analysis. Under pressure to catch up to Airbus to approve its new 737 Max jets, Boeing turned in a safety assessment to the FAA that was riddled with errors, the Times reported.

The Chicago-based company is under increased scrutiny after two of its Max 8 jets crashed within five months of each other. Last Sunday, Ethiopian Airlines Flight 302 crashed a few minutes after taking off from Addis Ababa, killing all 157 people on board. This followed the October crash of Lion Air Flight 610 in Indonesia that killed all 189 passengers and crew. On Wednesday, the FAA ordered the plane to be grounded.

In the aftermath, investigators have focused on an anti-stall feature known as Maneuvering Characteristics Augmentation System (MCAS). A preliminary report from Indonesian investigators indicates that Lion Air 610 crashed because a faulty sensor erroneously reported that the airplane was stalling. The false report triggered MCAS, which tried to point the aircraft’s nose down so that it could gain enough speed to fly safely.

Boeing’s safety assessment of MCAS understated the system’s power and failed to account for how it could reset itself after each time a pilot responded. Black box data retrieved after the Lion Air crash indicates that a single faulty sensor triggered MCAS multiple times during the deadly flight. This led to a struggle between the pilots and the plane, as the system repeatedly pushed the nose of the plane down and the pilots wrestled with the controls to pull it back up before the final crash.

The Justice Department probe of the FAA certification process is highly unusual

The Justice Department probe of the FAA certification process is highly unusual, and it could raise questions about President Trump’s involvement. According to The Washington Post, Trump was the “key arbiter” in the decision over the Boeing 737 Max 8 and 9 jets. Trump participated in phone conversations with top Boeing executives and other stakeholders, offering his thoughts on the aviation industry. But since then, he has faced criticism that his over-involvement stymied the FAA from acting sooner.

Boeing declined to comment on the subpoena from federal investigators. In a statement, the company said: “The FAA considered the final configuration and operating parameters of MCAS during MAX certification, and concluded that it met all certification and regulatory requirements.”

On Sunday, Boeing CEO Dennis Muilenburg issued a statement describing steps the company was taking to update its technology. “While investigators continue to work to establish definitive conclusions, Boeing is finalizing its development of a previously-announced software update and pilot training revision that will address the MCAS flight control law’s behavior in response to erroneous sensor inputs,” Muilenburg said.