The flames were on Sierra Road, along the north side of the park.
In the hills of the Sierra Vista Open Space Preserve, there is nothing natural to spark a fire.
"There's no buildings around. There's been fireworks activity up here, you know, before Fourth of July, after Fourth of July," SJPD Captain Matt Paiss said.
Paiss suspects someone ignited the grass with fireworks around 2:30 a.m.
"This area here, on a different day with different weather conditions, could have been far more serious. This is a very, very hazardous area. Having a fire get down into Alum Rock Park is one of the things that we're most concerned about," Paiss said.
Ten acres were charred before a crew of 70 firefighters could contain the flames.
Investigators are still working to determine whether it was accidental or intentional. "Just because someone is playing with fireworks is not necessarily intending to start a fire," Paiss said.
There are warning signs everywhere about the fire danger. Smokign isn't allowed because the grass is prone to ignition.
Fireworks are illegal in San Jose.
Firefighters will be checking for hotspots throughout the day.