Police are investigating an incident where a lit firecracker was...

Police are investigating an incident where a lit firecracker was thrown at the Alverta B. Gray Schultz Middle School on Greenwich Street in Hempstead and five people were hurt, police said. (July 5, 2011) Credit: Howard Schnapp

Five adults were injured after a 15-year-old boy lit a firework mortar in a Hempstead middle school during summer school registration Tuesday, police said.

Police said the teen, who has been charged as an adult with first-degree arson, lit and threw down the mortar about 11 a.m. in the media center at Alverta B. Gray Schultz Middle School on Greenwich Street.

The mortar exploded three different times causing damage to the floor, walls and ceiling, police said.

The five people taken to the hospital were ages 18 to 47 and included a pregnant woman. They had nonlife-threatening injuries, including burns, leg fractures, and knee and stomach pain, police said. The injuries occurred when people who had gathered for registration ran in panic from the building and were trampled by others, police said.

Nassau County Police Department Arson/Bomb Squad detectives investigated the incident.

School and district officials were not immediately available for comment.

The teen is to be arraigned Wednesday in First District Court in Hempstead.

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