Investigators say the most recent fire happened around 4:30 p.m. Monday on the grounds of the nonprofit center, which also includes the historic 1924 mansion, a popular venue for weddings and special events.
Officials say the first of seven fires began in January. They started small, damaging a garden cart and later a couple of trash cans. But on Sunday, someone started a fire under the group's minivan, totaling it.
"They literally tried to set fire to the gas tank on that. They were trying to blow it up," said Adam England, director of operations at the Muckenthaler Cultural Center.
The next day was the worst fire yet. A historic piano, played by thousands of musicians over the years, was destroyed.
"It's sad. It's like seeing an old friend, just old friend, being lost. A lot of beautiful performances have been on that," said England.
The blaze also damaged part of the amphitheater stage. The fires have put many people on edge.
"Every time we hear a siren going by on the street outside, we all get really nervous," said England.
The center recently installed security cameras. Muckenthaler officials say an unidentified young woman is seen heading up to the amphitheater stage right before the fire. She's seen carrying something under her arm and a backpack.
"She goes up there and when she comes back, she doesn't have the items she's carrying," said England.
Police are looking at surveillance video but say they have not yet identified any suspects. Many are left wondering why someone would do this.
"That has made me absolutely sick. The last two nights, I couldn't even sleep to think that somebody would do something like that to hurt an organization like us, who's here for the community. This is a little gem here," said Ann Milazzo of Anaheim.
Anyone with information about the fires was asked to call Fullerton police at (714) 738-6800.