A bomb ripped apart a bus in Volgograd on Monday, killing 14 people in the second deadly attack blamed on suicide bombers in the southern Russian city in 24 hours and raising fears of Islamist attacks on the Winter Olympics.

President Vladimir Putin, who has staked his prestige on February’s Sochi Games and dismissed threats from Chechen and other Islamist militants in the nearby North Caucasus, ordered tighter security nationwide after the morning rush-hour blast.

Investigators said they believed a male suicide bomber set off the blast, a day after a similar attack killed at least 17 in the main rail station of a city that serves as a gateway to the southern wedge of Russian territory bounded by the Black and Caspian Seas and the Caucasus mountains.