The death toll in the suicide attack in Russia's Volgograd city Monday rose to 15 even as President Vladimir Putin ordered that security be tightened all over the country.
"Twelve people died at the blast site, two people died on the way to the hospital, one more person died at the hospital," a source in Volgograd's medical services said after the blast in a trolley bus in the city Monday morning.
Twenty seven people, who were wounded, are undergoing treatment at hospitals in the south Russian city.
Monday's was the second terrorist attack in two days in the city even as Russia was preparing to host the Winter Olympics at Sochi in February next year.
At least 17 people were killed and over 40 wounded in an explosion Sunday at the railway station in Volgograd. According to the latest reports, five people are on the critical list.
President Vladimir Putin has instructed the National Anti-terrorism Committee (NAK) to tighten security all round Russia and in the Volgograd region, NAK sources reported Monday.
The NAK's operations headquarters has ordered additional measures to tighten security in crowded areas in Volgograd.
"The operations headquarters ask residents of the Volgograd region and its guests to have documents which certify their identity", the NAK reported.
Meanwhile, Vladimir Markin, a spokesman for the Investigation Committee of Russia, told ITAR-TASS that Monday's attack was carried out by a male suicide bomber.
He said that investigators were working at the scene of the explosion and doing a detailed examination of the area."
"Only the metallic frame is left of the trolley and the roof is practically absent," Markin said.
He said "the explosion was so powerful that it blew out window glass in a five-storey building opposite the site."
"By way of preliminary information, we can say now that the explosion was triggered by a male suicide bomber, since fragments of his body have been found and sent for a genetic study in order to establish his identity," Markin added.
He also confirmed a link between Monday's blast and that of Sunday.