Rumors that battle-hardened Chechen fighters from Russia's notorious Vostok Battalion are active in eastern Ukraine have been swirling for weeks.
They unexpectedly materialized on May 29 when dozens of heavily armed men identifying themselves as members of the Vostok Battalion stormed the separatists' headquarters
in central Donetsk, evicting the motley band of pro-Russian rebels that had occupied the building since March.
The brazen raid, conducted in broad daylight, has plunged the region into new uncertainty. The emergence of such a widely recognizable Russian military structure in eastern Ukraine has also raised questions about Moscow's role in the conflict.
So what is the Vostok Battalion and what is it doing in eastern Ukraine?
The Vostok ("East") Battalion was formed by Chechen warlord Sulim Yamadayev in 1999, at the onset of the second Chechen war.
Together with his four brothers, Yamadayev defected from the Chechen separatist insurgency in protest at its growing Islamization and rounded up a group of loyal fighters.
The newly formed Vostok Battalion remained stationed in Chechnya.
It answered directly to the Russian Defense Ministry's main intelligence directorate, the GRU, and was tasked with rooting out Arab jihadists fighting alongside local insurgents.
In 2008, the unit was dispatched to help pro-Russian separatists from South Ossetia in the Russian-Georgian war.
It was officially disbanded shortly after the war in what experts believe was a political move to end the scorching rivalry between "Vostochniki," as the battalion was colloquially known, and members of the "Kadyrovtsy," the feared militia controlled by Moscow-backed Chechen leader Ramzan Kadyrov.
Yamadayev's 2009 killing in Dubai sealed the Vostok Battalion's demise.
The unit, however, was not truly dissolved.
"It was never really broken up, it was re-profiled and incorporated into a Defense Ministry unit based in Chechnya," says Ivan Sukhov, a Russian journalist and North Caucasus expert.
Despite Russia's claims that it isn't involved in the eastern Ukrainian conflict, the emergence of a Vostok Battalion in Donetsk is not entirely surprising.
"We know there are well-trained North Caucasus units, formed on the basis of their ethnicity, that are ready for combat and have long been in reserve," Sukhov says. "They were used during the war with Georgia in 2008, and those in charge no doubt remember they have this resource at their disposal."
The battalion now flexing its muscles in eastern Ukraine, however, is unlikely to be an exact resurrection of the commando formed by Yamadayev 15 years ago.
"I think the heart of the unit is made up of veterans of the original battalion," says Mark Galeotti, a New York University professor and expert on Russian security affairs. "But it is clear that the present incarnation also includes non-Chechens and soldiers who did not fight in the earlier force."
The Vostok fighters in Donetsk say they want to put an end to the rebels' looting of groceries from local supermarkets.
Their raid on the separatists' headquarters, however, is widely seen as an attempt by a group of Moscow-connected separatists to rid the insurgency of ragtag elements and assert control over eastern Ukraine.
This group is led by Igor Girkin, who goes by the pseudonym "Strelkov" and commands the separatists' military operations in Slovyansk, and Aleksandr Borodai, the prime minister of the self-proclaimed "Donetsk People's Republic."
Girkin -- who the Kyiv government says is a Russian military intelligence officer -- has himself taken a tough stand against indiscipline within rebel ranks, recently ordering the execution of two looters.
The Donetsk raid was obviously aimed at shaming the militants who had established quarters there, analysts say.
Vostok fighters actually led Western journalists through the reclaimed building, vigorously breaking down doors and showing off the groceries, cigarettes, and alcohol looted by the previous occupiers.
This adds weight to the notion that Moscow, after tacitly fueling separatist unrest for weeks, is now eager to rein in the spiraling anarchy unleashed in eastern Ukraine.
Galeotti says the battalion was "clearly either directly created by Russian military intelligence or at the very least blessed by it."
"I think this represents an attempt to put in a force that is more disciplined but above all that looks to Moscow for orders," he adds.
Galeotti says Russia could also be seeking to rein in the potentially explosive enmity between different armed rebel factions.
But while the Vostok fighters seem cut out for the job, with their experience in underground guerrilla operations, their appearance considerably raises the stakes in Ukraine.
"The presence of these people in southeastern Ukraine," says Ivan Sukhov, "is a scandal."
RFE/RL's Russian Service correspondent Svetlana Pavlova contributed to this report