Mentioning fig leaf will evoke the story of Adam and Eve who covered their private parts with fig leaves. (For more information, see http://en.wikipedia.org/wiki/Fig_leaf.)
We can find fig leaves on some ancient statues, such as this statue of Mercury in the Vatican. (The image can be found on the same Wikipedia page mentioned above.)
Naturally, fig leaf has become a phrase for, as you said, something to hide something else that is often embarrassing (or considered morally wrong). To put it simply, it's a cover that is used to cover whatever you want to hide because you think it's embarrassing or you feel ashamed of it.
Here is our sentence in question:
The November 1984 killings were the result of targeted, one-sided attacks on Sikhs by mobs that could not even summon the fig leaf of a prior or imminent attack on Hindus as a justification.
The author appears to believe that the mobs should be ashamed of these killings (as indicated by "targeted, one-sided attacks").
And if the mobs tried to justify this result by covering it with a prior attack--or in their own words: "summon the fig leaf of a prior or imminent attack"--it just would not work.
In short, in their opinion, the killings were result from shameful acts, and the word fig leaf is used figuratively to refer to a prior attack, which to some might cover (justify) the acts, but for the author, it could not.