NEW YORK-- Officials at the Sept. 11 memorial in New York say one of their security guards shouldn't have stopped a North Carolina middle school choir from singing the national anthem.

A spokeswoman for the memorial says the guard didn't respond appropriately Wednesday when he silenced some 50 students from Waynesville in the middle of "The Star-Spangled Banner."

CBS New York reports that in the middle of their performance, the choir was approached by two security guards who told them to stop, because the singing could be considered a public demonstration.

Video posted to Facebook by an adult on the school field trip sparked outrage and led to an invitation to perform Monday on Fox News.

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Chorus teacher Martha Brown told Fox that one security guard at the memorial plaza had given the students permission to sing, but another guard told them to stop.

Groups wishing to perform at the Sept. 11 memorial are supposed to apply for a $35 permit. The North Carolina group did not.