Azerbaijan’s leader on Monday accused Moscow of carrying out a “cover up” over a passenger jet crash last month that claimed 38 lives, as relations sour between the two neighbors.

The Brazilian Air Force Laboratory has sent data from the flight recorders of an Azerbaijan Airlines plane to Kazakh investigators. The plane crashed on December 25 near the city of Aktau, Radio Liberty reports.

Brazilian investigators have reportedly begun analyzing black boxes of the Azerbaijan Airlines Embraer E190 involved in the crash. The news comes almost two weeks after the regional aircraft operating flight J28243 from Baku to Grozny crash-landed near the city of Aktau in Kazakhstan, killing 38 people.

The black box from the Azerbaijan Airlines crash in Kazakhstan, believed to have been caused by a Russian missile, is being sent to Brazil for investigation, Kazakh authorities reported on Dec. 29. Kazakhstan’s decision aligns with Azerbaijan's call for a transparent probe into the crash,

Kazakhstan's government told Euronews that it has decided to send the downed Azerbaijan Airlines flight recorders to Brazil for the full and transparent disclosure of the tragedy.View on euronews

In the crash’s aftermath, Azerbaijan has unleashed rare and stinging criticism of Russia, with the country’s president saying Moscow’s response has caused “surprise, regret and rightful indignation.”

Experts from investigation commission to begin studying decrypted data from crashed plane’s black boxes as soon as they arrive in Astana, says Kazakh Transport Ministry - Anadolu Ajansı

The voice recorders from the Brazilian-made plane were analyzed in Brasilia before the data was handed to Kazakh authorities. Meanwhile, Azerbaijan has directed unusually strong words towards Moscow.