Aug 13 (UPI) — Russian officials said Tuesday radiation levels at a military test site involved in an explosion last week were four to 16 times above natural background levels.

The explosion led to a spike in radiation in Severodinvsk, but later returned to normal, officials said. Rosgidromet, Russia’s watching weather service, said the spike was significant.

Leonid Bolshov, of the Russian Academy of Sciences’ Nuclear Safety Institute, told TASS the explosion offered no risk to the population of the Arkhangelsk region. Valentin Magomedov, one of the city’s civil defense department, said radiation in Severodvinsk was three times the permissible level, though.

U.S. officials increasingly believe that the explosion was part of tests for a new cruise missile charged by nuclear power, which could have contaminated the region. Russian media speculated that it could be the SSC-X-9 Skyfall, a nuclear-powered cruise missile Russian President Vladimir Putin bragged about last year.