  The holonomy algebra $\g$ of an $n+2$-dimensional Lorentzian manifold $(M,g)$
admitting a parallel distribution of isotropic lines is contained in the
subalgebra $\simil(n)=(\Real\oplus\so(n))\zr\Real^n\subset\so(1,n+1)$. An
important invariant of $\g$ is its $\so(n)$-projection $\h\subset\so(n)$, which
is a Riemannian holonomy algebra. One component of the curvature tensor of the
manifold belongs to the space $\P(\h)$ consisting of linear maps from $\Real^n$
to $\h$ satisfying an identity similar to the Bianchi one. In the present paper
the spaces $\P(\h)$ are computed for each possible $\h$. This gives the
complete description of the values of the curvature tensor of the manifold
$(M,g)$. These results can be applied e.g. to the holonomy classification of
the Einstein Lorentzian manifolds.
