Russia suffered one of its most serious losses in the 28-month Chechnya war when five commanders, including two top generals, were killed when their helicopter was shot down over the region.
Initial Russian reports attributed the explosion to "an act of terrorism," signaling involvement from Chechen rebels.
I nterfaxnews agency said a preliminary investigation of the wreck showed the chopper had been shot down by a surface-to-air missile.
The Mi-8 chopper's downing coincided with the fifth-year anniversary of the election of separatist Chechen leader Mr Aslan Maskhadov, who has vowed to remain president of the republic even though his mandate officially expired today.
However an FSB (Federal Security Service, ex-KGB) spokesman Mr Sergei Babkin later told Interfaxthat "the cause of the explosion had not been established as of yet."
There was no immediate claim of responsibility for the incident from any senior Chechen representatives.
The crash will deliver a severe blow to Russian President Mr Vladimir Putin, who has declared the Chechnya war as already won, saying federal troops are only in the region for policing purposes.
Russian news reports said that some of the most senior Russian commanders of the Chechnya campaign had been killed.
These included interior ministry General Mikhail Rudchenko, who oversees the southern Russian administrative region, and his deputy.
AFP