He is the criminal. In the preview from last time, he was saying ~.
ドラマなどの予告には"preview"を使えます。
"He is the criminal."=「彼は犯人です。」
"In the preview from last time"=「前回の予告では」
"he was saying ~ ."=「彼は○○と言っていました。」
Sentence in context:
"I think it's safe to assume that he is the criminal, based on what we saw in the preview."