By Laura Smith-Spark
Russian authorities are investigating six suspicious deaths and at least one car explosion this week about 300 miles from the site of next month's Winter Olympics, Russian state news agency RIA Novosti reported Thursday.
The deaths in southern Russia's Stavropol territory, which borders the province where the Olympics will be held in Sochi, have prompted security forces to conduct an anti-terrorism sweep there -- one of the latest moves to address security concerns before the games.
The bodies of three men and explosive material were found Thursday in a vehicle in an unspecified location in Stavropol province, RIA Novosti reported.
A day earlier, a vehicle containing a body exploded as police approached it in the same province, and two other bodies were found in other vehicles there Wednesday, RIA Novosti reported.
The latest reported discovery comes amid heightened concern about security following twin bombings on public transit in the southern city of Volgograd at the end of last month.
No group has claimed responsibility for those attacks, but suspicion has fallen on Chechen separatist groups.
Three people also died last month in a car bombing in the city of Pyatigorsk, in Stavropol territory.
A special exclusion zone went into force in Sochi on Tuesday, under which access to the Black Sea resort town is heavily restricted.
Security analysts have warned that terrorists targeting the games may try to strike elsewhere in Russia against softer targets.
Chechen warlord Doku Umarov, leader of the Islamist Caucasus Emirate group, last summer called on his followers to do what they could to disrupt the games.
CNN's Jason Hanna contributed to this report.
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