Indirect Proofs Help Please

Shelly, Susan, and Samantha are all working at an amusement park for the summer. The manager finds out that one of them left the key in the door to the security office. When he asked them who did it, they replied:

Shelly: Samantha did it.
Susan: Shelly did not do it.
Samantha: I didn’t do it.

The manager concluded that Samantha did it because she was accused and was denying it.

Use an indirect proof to show that the manager was wrong and that Samantha did not leave the key in the security office door.

Question

Indirect Proofs Help Please

Shelly, Susan, and Samantha are all working at an amusement park for the summer.  The manager finds out that one of them left the key in the door to the security office.  When he asked them who did it, they replied:
 
Shelly: Samantha did it.
Susan: Shelly did not do it.
Samantha:  I didn’t do it.
 
The manager concluded that Samantha did it because she was accused and was denying it.
 
Use an indirect proof to show that the manager was wrong and that Samantha did not leave the key in the security office door.

experimentx · Answer

Susan: Shelly did not do it => other than shelly did => leaves Susan, Manager, & Samantha
Samantha:  I didn’t do it. => removes Samantha => Susan, & Manager
Manager: most likely did not do it => Might be Susan or Himself

More than anything, I can Question is amusing

anonymous · Answer

Well,if you say it in that way you are assuming everybody is telling the truth which clearly isn't the case.In any case I agree the question is interesting

experimentx · Answer

lol ... we have a contradiction here: 

Shelly: Samantha did it..
Samantha:  I didn’t do it.

But nobody said nothing about Susan and Manager ... let's make up some interesting story.,,