Fourteen people were killed when a bomb blast ripped through a trolleybus early on Monday in the second deadly attack in the Russian city of Volgograd in two days, law enforcement authorities said.
Russian investigators said they believe a male suicide bomber carried out the attack.
"It is now possible to preliminarily say that the explosive device was set off by a suicide bomber - a man whose body fragments have been collected and sent for genetic testing," the federal Investigative Committee said in a statement.
The explosion came a day after a female suicide bomber killed at least 17 people in the main railway station of the southern city 40 days before Russia is to hold the Winter Olympics in the Black Sea resort of Sochi.
State TV footage showed the twisted, gutted remains of the blue-and-white trolleybus, its roof blown off and debris strewn around the street. Federal investigators said the blast was believed to have been a "terrorist act."
Russian President Vladimir Putin on Monday ordered law enforcement agencies to increase security in the southern city of Volgograd and nationwide, the Kremlin said.
Putin issued several instructions to a committee that coordinates counterterrorism efforts "to strengthen security Russia-wide and specifically in the Volgograd region," the Kremlin said. It did not describe the instructions.
The United States condemned the attack in Volgograd "in the strongest terms." A statement issued by the U.S. State Department said: "We send our sincere condolences to the families of the victims and stand in solidarity with the Russian people against terrorism of any kind."
The consecutive attacks underscored Russia's vulnerability to militants and will raise fears of a concerted campaign of violence before the Olympics, which start on February 7 in Sochi, about 690 kilometer (430 miles) southwest of Volgograd.
The Sochi Games are a major prestige project for President Vladimir Putin. Insurgents who want to carve an Islamic state out of southern Russia urged militants in a Web-posted video in July to use "maximum force" to prevent them from being held.
There was no immediate claim of responsibility for either of the attacks, which left dozens wounded in addition to the deaths. Authorities said 37 people were hospitalized after Sunday's attack and 23 were reported wounded on Monday.
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