The owners of the Caverns of Sonora in Southwest Texas are offering a reward for the return of a national treasure — part of a rare butterfly formation that took shape over thousands of years.

Robbed of their dazzling centerpiece, valued in excess of $500,000, on the Tuesday before Thanksgiving, cave owners are offering a $20,000 reward for its safe return.

They claim college-age visitors from the Houston area may have been responsible for the vandalism, which apparently involved the intentional distraction of a tour guide.

Sutton County Sheriff Joe Fincher declined comment on possible suspects.

"We're certainly continuing the investigation, and if it warrants, subjects will be arrested," Fincher said Thursday.

Also declining to discuss witness accounts and other evidence gathered in the past three weeks, the sheriff said, "we've got some subpoenas issued and we're going from there."

Fincher said he's eager to solve the criminal case.

"It's a pretty serious deal," Fincher said.

Cavern owners issued a statement late Wednesday expressing heartbreak over the loss of "one of America's greatest speleological treasures" to "an act of senseless destruction" on Nov. 21.

The owners said they believe a group of visitors "knowingly distracted a tour guide with the intent of destroying the cavern's most famous formation, known as the Butterfly, a bladed helictite."

Owners said they're "working with local, county and state officials to retrieve records related to visitors on the day the vandalism occurred.

A preliminary investigation revealed a visitor from the Houston area as a possible suspect."

In an interview, co-owner Seco Mayfield said the probe has focused on a group of 11 visitors, several of whom have been identified through campground records, credit cards and other means.

Although the site is a National Natural Landmark, it isn't covered by federal anti-vandalism laws because of its private ownership, Mayfield said. However, because of the estimated value of the loss, the vandalism may qualify as a felony theft under state law, Mayfield said.

The missing wing portion is 4 1/2 inches by 3 1/2 inches and could be reattached if not seriously damaged, Mayfield said.

"The Butterfly is our trademark," he said of the natural formation shaped by mineral deposits. "It's an integral part of our business."

Mayfield co-owns the caverns with his sister, Gerry Ingham, and each put up $10,000 for the reward. The National Speleological Society and other groups have pledged to the reward fund.

"We want to find these people and try and convince them to give it back or tell us what happened to it," Mayfield said.

"Regardless of what happens with the investigation, something that will probably come out of this is a coordinated effort by our industry and others to strengthen the laws that would apply," he added.

john.gonzalez@chron.com