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.

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.

SAO PAULO (Reuters) - The extraction of flight data from the Brazilian-made Azerbaijan Airlines plane that crashed in Kazakhstan last month was completed at a lab run by Brazil's air force, according to a statement released by the entity on Monday. The data from the Embraer plane was sent to the Kazakhstan authority investigating the crash.

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.”

Black boxes from plane crash will be delivered to Aeronautical Accidents Investigation and Prevention Center (CENIPA) in Brazil, says Kazakh Transport Ministry - Anadolu Ajansı

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.