。
She declared that she was innocent.
Literal
She [topic-は] [self [topic-は] innocent is] [quotative-と] declared.
断言する ('to assert categorically, declare without equivocation') is one of several formal verbs of communication that take quotative と. The embedded copula appears as plain だ rather than polite です — typical of indirect summarized speech, where the narrator settles on a single register rather than reproducing the speaker's exact wording. 自分 in the embedded clause refers back to the matrix subject 彼女.