Things the police/FBI can find -- matching spray paint mist on clothes, shoes; the can, which was probably found, traced to the store where it was bought and where the clerk or a paper trail may identify the buyer -- if the time of day the purchase was made is known, they will look for suspect's alibi. If the police are even marginally competent, they immediately impounded every campus dumpster and combed the whole place for evidence. They're not dealing with an especially clever suspect.
This does appear to be the case.