The Transportation Department confirmed Tuesday that its watchdog agency will examine how the Federal Aviation Administration certified the Boeing 737 Max 8 aircraft, the now-grounded plane involved in two fatal accidents within five months.

The FAA had stood by the safety of the plane up until last Wednesday, despite other countries grounding it.

Transportation Secretary Elaine Chao formally requested the audit in a letter to Inspector General Calvin Scovel III.

Chao, whose agency oversees the FAA, said the audit will improve the department's decision-making. Her letter confirmed that she had previously requested an audit but did not mention reports that the inspector general and federal prosecutors are looking into the development and regulatory approval of the jet.

RELATED: 5 things to know about grounding of Boeing 737 MAX planes

A Lion Air Max 8 crashed off the coast of Indonesia last October, and an Ethiopian Airlines Max 8 crashed this month near Addis Ababa.

Investigators suspect that incorrect sensor readings feeding into a new automated flight-control system may have played a role in the Indonesian crash, and the Ethiopian plane had a similar, erratic flight path.

Boeing began working on an upgrade to software behind the flight-control system shortly after the Lion Air crash. CEO Dennis Muilenburg said in recent days that the company is close to finishing the update and changes in pilot training to help crews respond to faulty sensor readings.

Acting FAA Administrator Daniel Elwell told House Transportation Committee members the upgrade will be done by next Monday, according to a person familiar with the briefing who spoke on condition of anonymity because they are not authorized to speak on the timing publicly. The FAA earlier required design changes to the flight-control system "no later than April."

The Associated Press reported Monday that the Justice Department and the Inspector General are probing development of the Max, according to a person briefed on the matter.

The Justice Department and the Transportation Department's inspector general are examining the way Boeing was regulated by the FAA, according to a person familiar with the matter and who asked not to be identified because the inquiry is not public.

Critics have questioned the FAA's practice of using employees of aircraft manufacturers to handle some safety inspections. FAA inspectors review the work of the manufacturers' employees, who are on the company payroll and could face a conflict of interest.

RELATED: Boeing CEO shares open letter after deadly 737 MAX crashes

A federal grand jury in Washington sent a subpoena to someone involved in the plane's development seeking emails, messages and other communications, the person told The Associated Press.

A report in The Wall Street Journal on Sunday said the inspector general was looking into the plane's anti-stall system. It quoted unidentified people familiar with both cases.

The Oct. 29 Lion Air crash killed 189 people, and 157 died in the March 10 accident involving an Ethiopian Airlines jet. Both accidents happened shortly after takeoff.

Other nations banned the Max 8 and a slightly larger model, the Max 9, in the days after the Ethiopian crash. The FAA and U.S. airlines that use the planes stood by the plane's safety until last week.

There are about 370 Max jets of various models at airlines around the world. American, Southwest and United have said the grounding of their Max jets have led to some canceled flights.

The plane is an important part of Chicago-based Boeing's future. The company has taken more than 5,000 orders and delivered more than 250 Max jets last year. Boeing still makes an older version of the popular 737, but it expected the Max to account for 90 percent of all 737 deliveries this year.