Ukraine’s State Security Service (SBU) has reason to believe that the pro-Russian separatists who captured three of its high-ranking Alpha Group counterterrorism officers, acted on a tip from infiltrators inside the spy agency, says Maryna Ostapenko, the SBU’s spokeswoman.

The SBU officers were captured in the city of Horlivka in Donetsk region on April 26. They were then transported to Sloviansk and are still being held there, the SBU believes. They were reportedly on a mission to conduct surveillance of and possibly arrest alleged Russian Military Intelligence Lieutenant-Colonel Igor Bezlier, who is suspected of involvement in the murder of a local city councilman.