Bob Cassilly, who turned a collection of unusual artifacts into City Museum, a top tourist attraction in downtown St. Louis, was found dead Monday morning at a former cement plant that was being turned into his next project, Cementland.

Cassilly, 61, was found in a bulldozer at Cementland, 9403 Riverview in north St. Louis. Police had no further details available immediately. Mayor Francis Slay said on Twitter shortly before noon that "The City has lost some of its wonder. RIP Bob Cassilly."

In a stark message, white type on a black background, the museum's website posted a message that said:

"City Musem is saddened by the loss of our founder and inspiration, Robert Cassilly. 1949-2011"

A spokesman for the St. Louis office of the Occupational Safety and Health Administration said it was investigating the scene of Cassilly's death to determine the cause and see whether any health or safety laws had been violated. He said the department has six months to complete a report on the matter and it was too soon to release any information about what its investigation has found.

As word of Cassilly's death spread, tributes to his work began to appear online.

A Facebook page was set up to remember Cassilly and feature some of his other works, including Turtle Park. And the Regional Arts Commission established a similar page, locating his various works of art throughout the area, including a bust of longtime alderman Red Villa and installations at the Zoo and the Butterfly House.

St. Louis Public Radio linked to an interview with Cassilly from 1999 when he appeared on the NPR program "Whad'ya Know?"

City Museum, whose slogan was "Where the imagination runs wild!," opened in 1997 in a 600,000-square-foot warehouse that formerly housed the International Shoe Company on 15th Street near Washington Avenue. It quickly became a top draw, with abandoned airplanes, elevated walkways and other items that the museum's website  terms "an eclectic mixture of children's playground, funhouse, surrealistic pavilion, and architectural marvel made out of unique found objects ... the very stuff of the city."

Cassilly, described on the site as a "classically trained sculptor and serial entrepreneur," worked with a team of artisans to create what he boasted had "urban roots deeper than any other institutions'."

In typical fashion, he said on the website:

"City Museum makes you want to know. The point is not to learn every fact, but to say, 'Wow, that's wonderful.' And if it's wonderful, it's worth preserving."

Cassilly's latest venture, Cementland -- which has been several years in the making and was running behind schedule -- is a 54-acre site at the old Missouri Portland Cement plant that a New York Times article described in a headline in 2007 as "one part cement, two parts whimsy, one odd park."

The story depicted Cassilly's vision this way:

"Imagine a park peppered with Mr. Cassilly's lively animal sculptures, but also with obsolete cement-making machinery grinding away, industrial silos and other remnants of the 54-acre former factory. Then add navigable waterways, waterfalls and beaches atop dirt hills."

It added:

"Pointing south, he rhapsodized about how downtown St. Louis would look from Cementland: 'In the afternoon, when the sun shines on the city, you get this nice reflection. You don't see all the trash and stuff. It's the best view of the city."

Panoramic photos of the ongoing work at Cementland can be found here and here; a Twitter feed from "cassilly crew," who describe themselves as "the personal build monkeys of creator Bob Cassilly," linked to them earlier this month.

City Museum is a regular stop for those seeking an offbeat experience in St. Louis amid more traditional fare like the Arch, the Cardinals, the Botanical Garden and the Zoo. It appeals to all ages with its variety - an aquarium, a funhouse, salvaged materials, a mega slide and MonstroCity, a huge outdoor jungle gym.

In a story about Cassilly and the museum on its 10th anniversary, the Post-Dispatch wrote that Cassilly "admits that he struggles to maintain his passion for City Museum and makes no promises about its future. 'You shouldn't assume things are going to last forever,' Cassilly said. 'It would be great if it all collapsed onto itself like Camelot. We would have had this brief shining ah-moment. But that's just the romantic in me.'"

Actually, City Museum almost collapsed, not under the weight of lack of interest but from a much more prosaic cause: divorce and other legal wrangling.

In 2002, after protracted, bitter proceedings between Cassilly and his former wife, Gail Cassilly, the board of directors at City Museum agreed to sell the attraction to Cassilly, who had guaranteed $1.6 million of the museum's debt, according to a story in the St. Louis Business Journal.

Gail Cassilly had been dismissed the previous year in a dispute over what direction the museum should take, the story said, and Cassilly had expressed displeasure over the museum's non-profit status.

As the couple's divorce dragged on, the museum had problems raising money. Cassilly eventually stopped working there, though he later returned and moved into an apartment in the building.

Despite the acrimony, Gail Cassilly expressed admiration for her ex-husband and pride in what they had built, telling the Post-Dispatch:

"I knew he was the master builder, and he knew I was the master organizer. I don't think it would have opened without that partnership. We really set the mark for fun."

The museum was the subject of other non-fun stories as well. In 2006, a jury awarded $100,000 to a woman who lost two fingers when she put her hand in the "Puking Pig," a metal tank that dumps about 150 gallons of water every 90 seconds or so. The jury said the woman was largely responsible for her injuries.

Last year, the family of a 10-year-old Kansas boy who fractured his skull after he fell 13 feet from the museum's outdoor jungle gym settled its case out of court; terms of the settlement were kept confidential.

Cassilly earned both bachelor's and master's degree in sculpture from Fontbonne after attending the Cleveland Institute of Art. Besides his many works in St. Louis, he created fiberglass hippos for Central Park in New York City; a giant giraffe for the Dallas Zoo; four bronze lions for Busch Gardens in San Diego; and a recreation of ancient stone ruins at Busch Gardens in Williamsburg, Va.