Who knocked over the monitor?

Melissa and Jessica were working on the computer along with their friends Sandy and Nicole.  Suddenly, I heard a crash and then lots of shouts.  I rushed in to find out what was going on, finding the computer monitor on the ground, surrounded with broken glass!  Sandy and Jessica spoke almost at the same time:

Jessica saying, "It wasn't me!"

Sandy saying, "It was Nicole!"

Melissa yelled, "No, it was Sandy!"

With a pretty straight face Nicole said, "Sandy is a liar."

Only one of them was telling the truth, so who knocked over the monitor? 

Find here who did it! 

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