Russia on Thursday launched a counter-terrorism operation after six bodies riddled with bullets were found in a region bordering that of Winter Olympic host city of Sochi, just weeks before the start of the Games.
Two districts in the southern Stavropol region were placed on high alert after the bodies were discovered in or close to parked cars, at least one of them apparently booby-trapped, in the rural area on Wednesday, the regional authorities said.
The Stavropol region borders the Krasnodar region where the Black Sea resort of Sochi is located.
"A counter-terrorism operation has been launched from midnight on January 9 in the Predgorny and Kirov districts of the Stavropol region," the regional administration said in a statement.
Rossiya 24 state television showed riot police patrolling a Kirov district filled with one-storey houses.
Three of the male victims have been identified, with one working as a taxi driver, it said.
On Tuesday, the authorities launched the most sweeping security operation in Olympic history, a month before Russia's first post-Soviet Games kick off on February 7.
Russia particularly fears attacks by Islamist militants from the North Caucasus during the prestigious event, after two suicide bombings at a rail station and in a trolleybus last month in the southern city of Volgograd killed 34.
No one has claimed responsibility for that attack, but Russian media said the attackers came from the restive North Caucasus region of Dagestan.
Under Russia's counter-terror legislation, security officials have sweeping powers to temporarily evacuate residents, stop cars and pedestrians, search premises and monitor phone calls and emails.
As police examined one of the cars in Stavropol, which contained the body of a local resident who had been shot in the head, an improvised explosive device (IED) went off around 20 metres (65 feet) away, causing no injuries, the regional interior ministry said in a statement.
Officers then defused another device nearby, Interfax news agency cited the FSB security agency as saying.
A security source told Interfax the two devices were packed with small metal elements such as screws to cause maximum injury and resembled those previously used by insurgents in southern Russia.
Two further bodies were found on the back seat and in the boot of Lada cars. Two more bodies were found in another parked car, while another lay nearby.
The Investigative Committee, which probes major criminal cases in Russia, opened probes into murder, arms trafficking and attempts on the lives of law enforcement officials.
It said Thursday that the series of murders, which took place within 24 hours, was to be treated as a single investigation.
A law enforcement source told Interfax that investigators were seeking three men from a village in the neighbouring Kabardino-Balkaria province.
In a recent attack in the Stavropol region, which is a popular tourist destination, three police officers were killed in a car bombing in a historic spa resort, Pyatigorsk, in late December.
Islamist militants who are seeking their own state in the North Caucasus have vowed to strike civilians in an effort to undermine the Games and President Vladimir Putin, whose prestige is linked closely to the event's success.
Putin called last month's Volgograd bombings an "abomination" and assured the nation he would "fight against terrorists until their total destruction".
Security measures introduced around Sochi on Tuesday include special permits for private cars or lorries to enter the city or access Olympic venues.
In unprecedented measures, Russia is also deploying drones to monitor the region and soldiers in armoured vehicles will patrol the city during the Games, which run until February 23.
Beyond Sochi, Moscow's international airports have introduced a ban to last throughout the Olympic and Paralympic Games on carrying any liquids in hand luggage.
The Russian Post Office has also introduced compulsory searches of any parcels sent to the Krasnodar region around Sochi.