US police are investigating after a four-year-old found methamphetamine inside his issued library book.

Police said a pupil from Shanksville-Stonycreek School District in Pennsylvania found the illegal drugs inside the book he was returning to the school last Wednesday.

According to police, teachers took possession of the substance and handed it over to police.

"We did follow our protocols when this information (came to light), and no students were put in danger throughout that process," Superintendent Samuel Romesberg said.

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"Our staff did a fabulous job addressing it according to our protocols.

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"In response to seeking additional information, we cannot comment on any aspect of any ongoing criminal investigation, other than to assure that in any given instance of which we are aware, our protocols have been followed, and we are not aware of any danger to any students of any kind.

"We do note that the Shanksville-Stonycreek School District has an agreement with the Indian Lake Borough Police, for onsite police presence as a general measure, to ensure student safety, and they may refer particular matters to the State Police as appropriate."