(QHA) - At least 14 people have been killed and 20 wounded in a suicide bomb attack on a packed trolleybus in Russia's Volgograd city, according to the Interfax news agency. Volgograd was on high alert on Monday following the latest attack that came a day after a suicide bombing at a train station in the city killed 17 people. "An explosion in Volgograd trolleybus is likely to be a terror attack, judging from the two previous attacks on a bus and a train station," said an official from the security services. The National Anti-Terrorism Committee said the trolleybus explosion at about 8:30 am local time was most likely caused by a bomb, but there were no further details. Russian investigators have opened a criminal probe into a suspected act of terror as well as the illegal carrying of weapons, according to Investigative Committee. The attack on Monday was the third on the city in the past three months. On October 21, a female suicide bomber blew herself up on a bus, killing six people.