In this paper we study a series of vertex operator algebras of integer level
associated to the affine Lie algebra Aℓ(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 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.Comment: 15 pages, LaTeX; final version, to appear in J. Algebr