According to reports, a powerful blast rocked a trolleybus around 8:10 a.m. during the morning rush hours on Monday.
Video footage and pictures posted online showed the trolleybus literally ripped apart with its roof blown out and bodies of passengers strewn on the road.
Investigative Committee spokesman Vladimir Markin told media that investigators believe a male suicide bomber carried out the attack, which he said was linked to the fatal bombing in the city on Sunday afternoon.
Markin reportedly said that remains of both suspected bombers, which were recovered at the scenes, are undergoing DNA testing.
On Sunday, about 17 people were killed and many injured in a deadly suicide attack by a female bomber in a Russian train station in Volgograd.
The female suicide bomber blew herself up at the entrance hall of the station, said officials.
Monday's blast is the third such strike in the city where in October another female suicide bomber had killed six people in a bus.
Russian President Vladimir Putin has ordered a tightening of security throughout the country after the attacks.
"He met with the heads of the Federal Security Service and Interior Ministry on Monday, sending FSB chief Alexander Bortnikov to Volgograd to help handle the aftermath," Russian news agency Ria Novosti reported.
Putin has also asked for daily reports from the National Anti-Terrorism Committee on measures to combat such attacks.
--IBNS (Posted on 30-12-2013)