By Aditya Kalra, Allison Lampert and David Shepardson

(Rtrs) -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.

Earlier this week, the United Nations aviation agency took the unusual step of offering India one of its investigators to provide assistance following the Boeing 787-8 Dreamliner crash killing 260 people in Ahmedabad on June 12.

Previously, the International Civil Aviation Organization has deployed investigators to help with certain probes, such as the downing of a Malaysian plane in 2014 and a Ukrainian jetliner in 2020, but those times the agency had been asked for assistance.

ICAO had asked for the investigator who was in India to be given observer status, but Indian authorities refused the offer, the sources said. The news was first reported on Thursday by the Indian news channel Times Now.

India's Aircraft Accident Investigation Bureau (AAIB), which is leading the probe into the world's deadliest aviation accident in a decade, did not return a request for comment. ICAO was not immediately available for comment. 

India's civil aviation ministry said on Thursday that investigators downloaded flight recorder data around two weeks after the crash.

Previously, safety experts had questioned a lack of information about the probe, including the status of the combined black box unit recovered on June 13, along with a second set that was found on June 16.

Questions were also raised on whether the recorders would be read in India or in the U.S. since the National Transportation Safety Board is participating in the investigation. The Indian government held only one press conference on the incident, and no questions were taken.

Under international rules known throughout the industry by their legal name "Annex 13," the decision of where to read flight recorders should be made immediately in case the evidence obtained could avert future tragedies.

Earlier this week, an Indian aviation ministry official who declined to be named said the department has been "following all the ICAO protocols." The official added that media representatives have made updates on important events.

Most air crashes are caused by multiple factors, with a preliminary report expected about 30 days after the accident.

(Aditya Kalra in New Delhi, Allison Lampert in Montreal and David Shepardson in Washington; Editing by David Gregorio)