“This may well be the work of Chechnyan terrorists.”
When it happened in 1999 (apartment bombings) the FSB ( successor to KGB) was responsible:
http://old.nationalreview.com/comment/comment-satter043002.asp
The first blast happened very near the current FSB headquarters.
Too near, IMO, to have been a deliberate act by the FSB itself.
On the other hand, the feds were mysteriously cleared out of the Murrah building, weren’t they?