POLTAVA, Ukraine — After four years of investigation by the German police, the F.B.I. and other crime-fighting agencies around the world, heavily armed security officers stormed an apartment in the central Ukrainian town of Poltava. After a brief exchange of gunfire, they captured their prey: the man suspected of leading a cybercrime gang accused of stealing more than $100 million.

The arrest of Gennadi Kapkanov, 33, a Russian-born Ukrainian hacker, and the takedown of Avalanche, a vast network of computers he and his confederates were accused of hijacking through malware and turning into a global criminal enterprise, won a rare round of applause for Ukraine from its frequently dispirited Western backers.

By the following day, however, Mr. Kapkanov had disappeared.

A judge in a district court in Poltava turned down a prosecution request that he be held in preventive custody for 40 days, and ordered him set free. Mr. Kapkanov has not been seen since.

Whether Mr. Kapkanov’s flight was the result of corruption, incompetence or a mix of the two has not been clearly established. The prosecutor general in Kiev, Ukraine’s capital, threatened to fire the local prosecutor but backed off when it became clear that the case had been handled by one of his own deputies.