Investigators in India are studying the black boxes of a Boeing 787 Dreamliner after recovering them from the aircraft wreckage to ascertain the cause of last week’s plane crash that left at least 270 people dead.

India would not allow a UN investigator to join a probe of a crashed Air India jet that some safety experts had criticized for delays in analysis of crucial black box data, two senior sources familiar with the matter told Reuters.

Analysis of flight recorder data from the deadly Air India crash two weeks ago was underway in India after investigators successfully retrieved the information from the two black boxes on board.