1. When we arrived at the murder scene, it had been cordoned off by the police.2. The police cordoned off the scene of the crime so that passers-by could not get close.
cordon offcordon off 指在某個地方拉起布條、繩子等，讓別人知道此地方禁止進入現場拉起了禁止進入的布條 = This area has been cordoned off.
The janitor neglected his duty on the day the robbers boke into the school.