When Sophocles' Oedipus the King opens, we find the people of Thebes suffering under the ravages of a terrible plague. Oedipus sends his brother-in-law Creon to Delphi to consult the oracle about a remedy for the situation. When Creon returns, he reports that the oracle has told him that the killer of Laius, the previous king, is still present among the Thebans. To rid themselves of the plague, Creon tells Oedipus that they must drive Laius' killer from their city.
Upon hearing this, Oedipus wonders why the Thebans did not try to investigate Laius' death at the time it happened.
When the ruling king had fallen in this way,
what bad trouble blocked your path, preventing you
from looking into it? (Ian Johnston translation)
Creon, in turn, responds that at the time the Thebans were also faced with another problem, the Sphinx, whose death, ironically, Oedipus subsequently brought about by solving the monster's riddle.
It was the Sphinx—
she sang her enigmatic song and thus forced us
to put aside something we found obscure
to look into the urgent problem we now faced.