DENVER, Colorado - Investigators are trying to determine why a woman caused $10,000 worth of damage to a large expressionist painting at the Clyfford Still Museum by punching and scratching it, then removing her pants and sliding down the artwork.

Carmen Tisch, 36, faces charges of criminal mischief in the Dec 29 attack on the painting, said district attorney spokeswoman Lynn Kimbrough.

The painting, referred to as 1957-J-No 2, is valued at more than $30 million. The large montage of black, white and burnt orange swaths with a sliver of yellow is from Still's middle period.

Museum officials said they believe security is adequate for the facility and that they regularly evaluate security to protect the collection and visitors. Museum spokeswoman Regan Petersen said in a statement that its guards "acted swiftly and appropriately; the police were summoned immediately and the offender was taken into custody".

Denver acquired the collection in stiff competition by promising to build a facility for the paintings and sculptures. Still, who died in 1980, specified in his will that his estate had to go to an American city willing to establish a permanent museum for his work. He was one of the first abstract expressionist artists following World War II.

The museum has raised $32 million in private donations for its building, endowment and operations, Petersen said. The city of Denver also contributed about $99 million from an auction of four Still works.

Visitors touring the gallery on Thursday said they were horrified by the attack. Rachel Gelbman and Christine Shaw, of Denver, said they had seen the painting at the Denver Art Museum and noticed it was missing, replaced by a similar painting from the 1956-58 era.

To them, it wasn't the same.

"What would possess someone to do that?" Gelbman said as security guards roamed the building.

Associated Press