  Let $\pi$ be a set of primes such that $|\pi|\geqslant 2$ and $\pi$ differs
from the set of all primes. Denote by $r$ the smallest prime which does not
belong to $\pi$ and set $m=r$ if $r=2,3$ and $m=r-1$ if $r\geqslant 5$. We
study the following conjecture: a conjugacy class $D$ of a finite group $G$ is
contained in $O\pi(G)$ if and only if every $m$ elements of $D$ generate a
$\pi$-subgroup. We confirm this conjecture for each group $G$ whose nonabelian
composition factors are isomorphic to alternating, linear and unitary simple
groups.
