Why is it die Investigation?

Investigation ends in -ion, and almost all nouns ending in -ion are feminine: in a German dictionary, 94% of them take die.

All nouns ending in -ion
Sense 1
die

Meaning

[1] Targeted search for information research

[1] gezielte Suche nach Information; Nachforschung

Examples

[1] The investigations are unfortunately unsuccessful

[1] Die Investigationen sind leider ohne Erfolg geblieben.

Declension

1 Singular Plural
Nominativ die Investigation die Investigationen
Genitiv der Investigation der Investigationen
Dativ der Investigation den Investigationen
Akkusativ die Investigation die Investigationen

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary