SAN DIEGO -- Students at a middle school here were sent to the playground for their safety as bomb experts swept the campus after a small explosion early this morning, school officials said.

No one was hurt and there was no major damage at Wagenheim Middle School in the Mira Mesa area, spokesman Jack Brandais said.

"This is purely precautionary," he said.

Within two hours of the blast that began the school day, authorities said a 12-year-old boy had confessed to placing a homemade device in a trash can and implicated four other boys. Although the device produced a large "kaboom," authorities said even the trash can was intact.

Authorities initially said the explosion came from a device that used dry ice. Arson investigators later determined it was another substance that can create a small explosion, according to the San Diego Fire Department. They did not identify the substance.

Police conducted a search of the campus for other devices, they said.

The incident comes just three days after a pipe bomb damaged the front of the federal courthouse in downtown San Diego. No one was injured in that incident, which occurred early Sunday and is being investigated by the FBI.

tony.perry@latimes.com