Police detained dozens of people on Tuesday in sweeps through the Russian city of Volgograd after two deadly attacks in less than 24 hours that raised security fears ahead of the Winter Olympics.
A man wounded when a bomber set off a blast in the city's railway station on Sunday died overnight, bringing the toll in that attack to 18. Regional governor Sergei Bazhenov said 16 died in a trolley bus bombing on Monday.
There was no indication that any of those held was connected to the attacks.
Nobody has claimed responsibility for the blasts, but they underscored vulnerability to bombings and raised fears of attacks by Islamist insurgents whose leader has called on militants to prevent Russia hosting the Olympics in February.
Mourners laid flowers at the site of the suicide bombing that tore the bus apart.
"I'm frightened," said Tatyana Volchanskaya, a student in Volgograd, 400 miles northwest of Sochi, the Black Sea resort where the Winter Games start on February 7. She said some friends were afraid to go to shops and other crowded places.
The attacks posed a challenge to President Vladimir Putin, who oversaw a war that drove rebels from power in Chechnya over a decade ago but has been unable to quell the Islamist insurgency that erupted in its wake.