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The lawyer strongly asserted his innocence.
Literal
Lawyer [topic-は] his not-guilty [obj-を] strongly asserted.
主張する ('to assert, to claim, to maintain') is the formal verb for making a legal or argumentative claim. 強く is the adverbial form of 強い ('strong'), describing the manner of assertion: 'strongly asserted.' 無罪 ('not guilty') as the object specifies what is being claimed — the verdict-worthy position rather than abstract innocence.