Russian investigators on Thursday were trying to determine who killed five men whose bodies were found in four cars in the southern Stavropol region close to the volatile Caucasus Mountains, a regional security spokesman said, Reuters reported. An unidentified explosive device was also found near one of the vehicles, said a spokesman for Russia's Federal Security Service in Stavropol. Russia has put security forces on combat alert in the southern Stavropol region following the killings. The country has already tightened security before next month's Winter Olympics in Sochi and is on high alert after suicide bombers killed at least 34 people in separate attacks in the southern city of Volgograd last month. About 37,000 personnel are now in place to provide security at the Sochi Games, Russian officials said.