CORNELIUS, N.C. --Warrants show more than a dozen teen girls at a North Carolina high school have been extorted for nude photos.

Multiple media outlets reported that Charlotte-Mecklenburg police are trying to find out who tried to extort more nude photos from the victims, all students at Hough High School in Cornelius.

The investigation says at least 16 teens ranging in age from 14 to 18 have been targeted.

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The warrant says the person sent a picture directly to each victim using photos the girls had taken of themselves and threatened to post the photos on websites unless the girls sent more pictures.

Another investigation began in February after police said dozens of nude photos of Charlotte-area teen girls began circulating on a website.

Police said the pictures were all voluntarily made.