Questions & Answers about Banken undersøger sagen.
Because -en is the definite article attached to the noun.
- bank = a bank
- banken = the bank
Danish often puts the at the end of the noun instead of using a separate word like English does.
For the same reason: -en makes the noun definite.
- sag = a case / a matter
- sagen = the case / the matter
So both nouns in the sentence are definite:
- banken = the bank
- sagen = the case