Two Russian monitoring stations designed to detect nuclear radiation went silent soon after the explosion at a missile test site this month, spurring concerns among observers that the Russian government is trying to restrict evidence of the accident.

Two days after the explosion of a suspected nuclear-powered cruise missile undergoing testing Aug. 8, two monitoring stations nearest the site of the accident stopped transmitting data, Lassina Zerbo, who heads the Comprehensive Nuclear Test Ban Treaty Organization, told The Wall...