This paper discusses the behavior of the Ricci flow at the first singular time of one solution for the Ricci equation \[ \frac{\partial}{\partial t} g_{ij}=-2R_{ij} \] on a smooth, compact and \(n\)-dimensional Riemannian manifold \(M\). If the flow has uniformly bounded scalar curvature and develops type I singularities at \(T\), it is shown that suitable blow-ups of the evolving metrics converge in the pointed Cheeger-Gromov sense to a Gaussian shrinker by using Perelman’s \(\mathcal W\)-functional. If the flow has uniformly bounded scalar curvature and develops type II singularities at \(T\), it is shown that suitable scalings of the potential functions in Perelman’s entropy functional converge to a positive constant on a complete, Ricci flat manifold. As a consequence, 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 \(T\).