Comment: Yes, But That Requires Re-assembly

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In reply to comment: #4: Usually removing one (see in situ)

Yes, But That Requires Re-assembly

Looking at the picture of the completed bomb, the handle is an integral part of the device and would require it to be screwed back into the pressure cooker. That means the perpetrator would have had to complete the final step of handle assembly i.e., take out the vessel, put on the handle, put it back into the backpack and leave it.

This would have looked suspicious and someone should have seen this being done.