American intelligence officials are racing to understand a mysterious explosion that released radiation off the coast of northern Russia last week, apparently during the test of a new type of nuclear-propelled cruise missile hailed by President Vladimir V. Putin as the centerpiece of Moscow’s arms race with the United States.

American officials have said nothing publicly about the blast on Thursday, possibly one of the worst nuclear accidents in the region since Chernobyl, although apparently on a far smaller scale, with at least seven people, including scientists, confirmed dead. But the Russian government’s slow and secretive response has set off anxiety in nearby cities and towns — and attracted the attention of analysts in Washington and Europe who believe the explosion may offer a glimpse of technological weaknesses in Russia’s new arms program.

Thursday’s accident happened offshore of the Nenoksa Missile Test Site and was followed by what nearby local officials initially reported was a spike in radiation in the atmosphere.

Late Sunday night, officials at a research institute that had employed five of the scientists who died confirmed for the first time that a small nuclear reactor had exploded during an experiment in the White Sea, and that the authorities were investigating the cause.