Gāi'àn de zhǔyào zhèngjù shì yī duàn jiānkòng lùxiàng.
The main evidence in this case is a segment of surveillance footage.
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该 is a formal demonstrative meaning 'the aforementioned' or 'this/that (previously mentioned)'. 该案 is a formal legal/journalistic term; in everyday conversation one would simply say 这个案子 or 这个案件.
