MOSCOW — Twin terrorist attacks in the city of Volgograd within 24 hours injected new urgency on Monday into Russia’s long, ruthless effort to contain a diffuse Islamic insurgency on its southern border, one nominally led by a veteran, battle-scarred Chechen often called Russia’s Osama bin Laden.

The attacks, coming only six weeks before the opening of the Olympics just 400 miles away, sowed widespread fear across the country. On Monday morning, a suicide bomber gutted a crowded electric trolley bus in Volgograd, killing at least 15 people and wounding dozens more.

The bomb exploded during the morning rush hour on a street only a little more than a mile and a half from the city’s main railroad station, where a similar attack killed at least 17 on Sunday.

The investigation into the bombings is just getting underway, but the attention of the Russian security services is already focused on the republic of Dagestan, which has become the hub of Muslim separatist violence in recent years, and on connections to the insurgent leader, Doku Umarov. He is a mysterious, almost mythical figure who fought in both Chechnya wars, which began nearly two decades ago and have come to symbolize the radicalization of a movement that began as a struggle for independence.