Another deadly blast has struck the southern Russian city of Volgograd, killing at least 10 people and further highlighting Russia’s security challenges as it readies to host the Winter Olympics in less than six weeks.

An explosion hit a trolleybus during the morning rush hour Monday, taking place the day after a blast at Volgograd’s main train stationkilled 17 people and wounded at least 35 others.

Like Sunday’s attack, the blast Monday was a terrorist act, Vladmir Markin, a spokesman for the country’s federal investigation agency, told the state-run news agency RIA Novosti.

No one claimed responsibility for the explosions. But they come several months after the leader of a Chechen separatist group pledged violence to disrupt the 2014 Sochi Winter Olympics.

The approaching Olympics

Video footage from the scene Monday showed the twisted shell of the blue trolleybus, with debris spread around it. The impact of the blast blew out the roof of the bus, as well as windows of several nearby houses.