The US Transportation Department's Inspector General has opened an investigation into the Federal Aviation Administration's (FAA) approval of Boeing's 737 Max planes which were involved in two deadly crash just five months apart, The Wall Street Journal said in a report.
The Journal's report on Sunday comes a week after the 737 Max planes were grounded for an indefinite period globally following the two deadly accidents involving the aircraft model, CNN reported.
The first accident was the March 10 crash of the Nairobi-bound Ethiopian Airlines flight 302 that took place just six minutes after it took-off from Addis Ababa, killing all 157 people on board.
The second crash occurred last October when a Lion Air Boeing 737 Max 8 - the same type of plane - plunged into the Java Sea minutes into the flight from Jakarta and killed all its 189 people.
The investigation will be focused on an automatic safety system implicated in the October crash, CNN quoted the Journal as saying citing a US government official.
It is unknown if the Ethiopian Airlines crash will play any role in this investigation or not.
The Transportation Department is yet to comment on the development.
Following the probe announcement, the FAA said in a statement, that its "aircraft certification processes are well established and have consistently produced safe aircraft designs".
Meanwhile, both the crashes, which occurred five months apart, remain under investigation by the Indonesia and Ethiopia governments, with the assistance from the US and Boeing.
The Ethiopian Minister of Transport said on Sunday that preliminary data recovered from the black boxes showed "similarities" between the two accidents.
A preliminary report of the Lion Air crash revealed that the pilots fought with the automatic safety system, known as the manoeuvring characteristics augmentation system (MCAS), for control of the jet.
Meanwhile, Boeing announced on Monday that it was working with the FAA to finalise a software update related to the MCAS system to make the planes safer. The company plans to have the update mandated no later than April, CNN reported.
"We also continue to provide technical assistance at the request of and under the direction of the National Transportation Safety Board, the US Accredited Representative working with Ethiopian investigators," Boeing CEO Dennis Muilenburg said in a statement.
According to Muilenburg, the company "continues to support the investigation, and is working with the authorities to evaluate new information as it becomes available".
"Safety is our highest priority as we design, build and support our aeroplanes. As part of our standard practice following an accident, we examine our aircraft design and operation, and when appropriate, institute product updates to further improve safety," he added.
