On 4 June 2016, a Boeing 737-800 instructed to climb from FL340 to FL380 by the controller of one sector in Bulgarian upper airspace came into sufficiently close proximity to an Airbus A320 under the control of a different sector controller to trigger co-ordinated TCAS RAs. Separation was eventually restored after the 737 followed its RA despite the A320, which had already deviated from its clearance on the basis of a prior TCAS TA without informing ATC, ignoring their RA. The Investigation found that the root cause of the conflict had been inadequate coordination between two vertically separated ATC sectors.