  We study groups of some virtual knots with small number of crossings and
prove that there is a virtual knot with long lower central series which, in
particular, implies that there is a virtual knot with residually nilpotent
group. This gives a possibility to construct invariants of virtual knots using
quotients by terms of the lower central series of knot groups. Also, we study
decomposition of virtual knot groups as semi direct product and free product
with amalgamation. In particular, we prove that the groups of some virtual
knots are extensions of finitely generated free groups by infinite cyclic
groups.
