Learn Turkish: Şirketin zararlı kimyasallar kullandığı resmî olarak belgelenmeli. - That the company used harmful chemicals must be officially documented.

Because the entire clause Şirketin zararlı kimyasallar kullandığı ("That the company used harmful chemicals") acts as the subject of the main verb belgelenmeli. Sentence subjects take the nominative case, meaning they don't get an extra case suffix. The -ı on kullandığı is purely the possessive suffix linking it to şirketin.

Why is "the company" written in the genitive case (şirketin) if it's the one performing the action?

In Turkish factive clauses made with -dık or -(y)acak, the "doer" of the action takes the genitive case, while the verb takes a possessive suffix. You can think of the grammar here literally as "The company's using of harmful chemicals".

Can you break down the suffixes on the final verb "belgelenmeli"?

It starts with the noun belge (document). Adding -le turns it into the verb belgele- (to document). The -n makes it passive, belgelen- (to be documented). Finally, -meli adds the obligation meaning, creating "must be documented".

Why do we need "olarak" after "resmî"?

Resmî is an adjective meaning "official". To use it as an adverb modifying the verb ("officially"), we add olarak (literally "being" or "as"). This is the standard way to turn many adjectives and nouns into adverbs of manner in Turkish.