Update: The fire is now being investigated by both the Orange County Fire Authority and the Sheriff's Department "to find out who started the fire and why," fire spokesman Marc Stone said.
The fire cost an estimated $10,000 in damages.
School remains open, with the playground area cordoned off, Stone said.
A suspicious playground fire has reduced a Ladera Ranch preschool's play equipment to "melted goo," Stone said.
The fire, which was reported Sunday night at Pacific Preschool at 29151 Sienna Parkway, may have been started intentionally, Stone said.
And what did it do to the playground made of composite plastic?
"What I saw was just a big pile of melted goo," he said. "It looked like if you left a popsicle on a sidewalk on a hot day."
A preliminary report will most likely be released later today, Stone said.