Investigators said at least one of the blasts at the checkpoint in Dagestan, a mostly Muslim area bordering Chechnya, was a suicide bombing, The New York Times reported.
The first bombing at the post near a highway outside of Makhachkala, Dagestan's capital, Thursday night was followed by an explosion set off after police and rescue workers arrived to aid victims, Russia's Investigative Committee said in a statement.
No one had claimed responsibility for the attack.
Russian media reports said the explosions tore through a natural gas pipeline, touching off a fire that prevented rescue workers from reaching many of the injured.
The attack occurred just days before the presidential inauguration of Vladimir Putin, who launched a war in Chechnya while he was prime minister in 1999, a move instrumental in his rise to power.
Russian Interior Minister Rashid Nurgaliyev said security measures will be tightened in the volatile North Caucasus region during World War II Victory Day celebrations Wednesday, RIA Novosti reported.
Dagestan has become the base of the Islamic insurgency in Russia's North Caucasus. Chechnya had two separatist wars in the 1990s and early 2000s.
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