Shacklebolt is of course speaking to the fact that Malfoy procured an illegal wand and casted an onslaught of murderous spells. "The fact that many Gala attendees remember a violent attack yet have such fuzzy memories of the night speaks to the unsanctimonious use of Dark Magic. Dark Magic, especially Ritual Magic, is often undetectable like that; its effects can not be seen in a Pensieve and its presence sometimes eclipses the minds of both the spell caster and the victims. It is my belief that Mr. Malfoy elicited the help of fellow student Hermione Granger. By some unsavory form of persuasion, he was able to use her to get the wand and participate in his attack that night at the Ministry. She may have been under the Imperius."