San Francisco -- The head of a children's art organization, arrested three weeks ago on suspicion of possessing child pornography, placed "offensive" tiles in murals he created at four city schools, San Francisco police said Friday.

About 100 "inappropriate" tiles have been found in murals at Sunset Elementary School and three other unnamed schools in the city, Police Chief Greg Suhr said.

Anthony Josef Norris, founder and director of Kid Serve Youth Murals, designed and worked on all the projects. The San Francisco man surrendered June 2 to the FBI after agents investigating a website known for child pornography traced the log-on name of "Spanky" to his home computer and discovered 600 pornographic images of children, according to an arrest warrant affidavit filed in federal court in San Francisco.

Also under investigation were murals Norris worked on at Old Mill Elementary School in Mill Valley, Global Family School in Oakland, and Bahia Vista Elementary School in San Rafael.

Police have not found inappropriate tiles at the Global Family School, said Troy Flint, a spokesman for Oakland Unified School District, who said the mural was checked Friday. Offensive images also were not discovered during inspections at the other two schools.

The confiscated mosaic tiles measure as small as 1-inch-by-1-inch, Suhr said. Many contain photographic negatives, inlaid behind glass. Others are drawings. Some were installed high above normal viewing levels.

"I think as an adult you look at these tiles and see possibly some sort of sexual connotation to them," said Cmdr. Mike Biel, who declined to describe the offensive images in detail.

Authorities began inspecting close to 40 murals, which are made up of tens of thousands of individual tiles, when the San Francisco Unified School District notified authorities after Norris' arrest, Suhr said.

So far, no evidence suggests that children were harmed in the making of the murals.

Norris founded Kid Serve Youth Murals about 12 years ago. He said in a 2009 interview with The Chronicle that his goal was to help youths from preschool to high school develop their design talent and create mosaic murals.