Why didn't the Thebans avenge the king's death in Oedipus Rex?

As 's
opens, the city of Thebes is wracked by a plague that has led to a host of
ills: the crops are blighted, the livestock barren, and the death toll mounts by the day.
Tormented by the suffering of his people, Kingdispatches his brother-in-law, , to seek an answer
to the city's terrible quandary from the Oracle of Delphi. The priestess tells him that the
pollution of the city will only be cleansed when the murderer of Laius, the city's former ruler,
is found and the king's death avenged.

As an impatient Oedipus questions
Creon about the death of Laius, in lines 119€“133 of the , the latter man tells as much as he
knows, explaining why the killers were never originally brought to justice:

Oedipus: And was there no one, no witness, no companion, to tell
what happened?

Creon: They were all killed but one, and he got away, so
frightened that he could remember one thing only.

Oedipus: What was that one
thing? One may be the key to everything, if we...