The fire broke out at about 8:35 p.m. inside the George Washington Elementary School, located at 347 William Street, in West Hempstead.
Witnesses told detectives they heard a loud explosion and several people running from the school. Soon afterward, witnesses saw smoke and flames coming out of the school.
One hundred firefighters from the West Hempstead Fire Department and five other fire departments were required to extinguish the flames. The building was vacant at the time of the fire.
The Nassau County Fire Marshall and the Arson/Bomb Squad were able to determined the fire was intentionally set.
The school suffered extensive fire and water damage. It is unclear if the damage will be repaired in time for the new school year in early September.
Anyone with any information is asked to call Crime Stoppers at (800) 244-TIPS. All calls will remain anonymous.