SIMFEROPOL, Aug. 13 -- The man had been delivering intelligence to Kyiv for three years, mapping the positions of Russian air defense systems across the Crimean peninsula while preparing to strike the railway lines that run through it. On Thursday, Russia's Federal Security Service said he would not complete either task.

Russia's FSB announced the arrest of a Russian national in Crimea who had operated as an agent for Ukraine's Security Service, supplying information about military deployments, equipment concentrations, and the location of air defense systems on the peninsula. The FSB said in a statement that the individual established contact with a Ukrainian intelligence handler and provided data at their direction over a period beginn...