Learn German: Wir verdächtigen den Chef der Tat, weswegen der Vorwurf ernst ist. - We suspect the boss of the deed, which is why the accusation is serious.

The verb verdächtigen (to suspect) strictly requires the genitive case for the thing the person is suspected of. In English we use "of", but in German, the genitive object der Tat handles this entirely on its own. Using a preposition here would be incorrect.

Is den Chef also affected by the genitive rule?

No, den Chef is in the accusative case. Verbs of accusing or suspecting like verdächtigen take a two-case structure: the person being suspected is the direct object (accusative), and the crime or action they are suspected of is the genitive object. The pattern is always "jemanden (Acc) einer Sache (Gen) verdächtigen".

How does weswegen work in the second half of the sentence?

weswegen means "which is why" or "because of which". It acts as a relative adverb that points back to the entire first clause to explain the consequence. Because it introduces a dependent clause, it forces the conjugated verb ist to the very end of the sentence.

Could I use deswegen instead of weswegen here?

Not with this exact punctuation. deswegen means "therefore" and is a main clause adverb, not a relative connecting word. To use it, you would need to start a new main clause, usually after a period: "Wir verdächtigen den Chef der Tat. Deswegen ist der Vorwurf ernst."