Washington (AP) - The US Federal Aviation Administration (FAA) has alleged charges against Boeing for alleged failures in connection with investigation into the approval of the Crisis Jet 737 Max.

FAA chief Steve Dickson, in a letter to Boeing chief Dennis Muilenburg published Friday by the agency, called for an "immediate explanation" as to why a "disquieting document" of supervisors was submitted months in arrears. Boeing did not comment at first.

According to a FAA statement, it is specifically news from two Boeing employees that a certain exchange of information between the Authority and the aircraft manufacturer on the initial approval of the 737 Max in 2016 after two crash-started crashes was described. Boeing had found this document months ago, but submitted to the Department of Transportation late Thursday, the FAA claims.

Both Boeing and the FAA have been seriously criticized by the 737 Max crashes, which killed 346 people in October and March. US investigators are investigating whether everything went right when it came to approving the planes. The FAA is said to have handed over significant parts of the Boeing certification itself.

Whether and when the 737 Max is allowed to withdraw is currently unclear. Investors responded nervously to criticism from the FAA on Friday, Boeing's shares fell in the US trade initially by just under four percent.