  In this paper we study a series of vertex operator algebras of integer level
associated to the affine Lie algebra $A_{\ell}^{(1)}$. These vertex operator
algebras are constructed by using the explicit construction of certain singular
vectors in the universal affine vertex operator algebra $N(n-2,0)$ at the
integer level. In the case $n=1$ or $l=2$, we explicitly determine Zhu's
algebras and classify all irreducible modules in the category $\mathcal{O}$. In
the case $l=2$, we show that the vertex operator algebra $N(n-2,0)$ contains
two linearly independent singular vectors of the same conformal weight.
