Fourteen people were killed when a bomb blast ripped through a trolleybus today in the Russian city of Volgograd.
The morning rush-hour bombing was the second deadly attack in the city in two days.
Yesterday, a suicide bomber killed at least 17 people in the main railway station of the southern city.
The attacks come less than six weeks 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.
Russian investigators believe that a male suicide bomber carried out today's 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.
Russian President Vladimir Putin has ordered law enforcement agencies to increase security nationwide, the Kremlin said.
Mr Putin issued several instructions to a committee that coordinates counterterrorism efforts "to strengthen security Russia-wide and specifically in the Volgograd region".
The consecutive attacks underscored Russia's vulnerability to militants.
They will also raise fears of a concerted campaign of violence before the Olympics, which start on 7 February in Sochi, about 690km southwest of Volgograd.
The Sochi Games are a major project for President 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 hospitalised after yesterday's attack and 23 were reported wounded today.