  In this paper, we study curvature behavior at the first singular time of
solution to the Ricci flow on a smooth, compact n-dimensional Riemannian
manifold $M$, $\frac{\partial}{\partial t}g_{ij} = -2R_{ij}$ for $t\in [0,T)$.
If the flow has uniformly bounded scalar curvature and develops Type I
singularities at $T$, using Perelman's $\mathcal{W}$-functional, we show that
suitable blow-ups of our evolving metrics converge in the pointed
Cheeger-Gromov sense to a Gaussian shrinker. If the flow has uniformly bounded
scalar curvature and develops Type II singularities at $T$, we show that
suitable scalings of the potential functions in Perelman's entropy functional
converge to a positive constant on a complete, Ricci flat manifold. We also
show that if the scalar curvature is uniformly bounded along the flow in
certain integral sense then the flow either develops a type II singularity at
$T$ or it can be smoothly extended past time $T$.
