1. seems to be more appropriate in the reported speech, changing this to that.
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Yankee
Hi YL
The sentence is not clear if you use 'that' rather than 'the robbery'.
Yoong Liat"Do not report this to the police," the robbers warned their victims.Hi Yoong,
1. The robbers warned the victim not to report that to the police.
2. The robbers warned the victim not to report the robbery to the police.