The brother of an imprisoned leader of a polygamous sect wants to double the amount of water available to the group’s compound in South Dakota, prompting concern by neighbors and law enforcement about a possible influx of members being displaced from an enclave on the Utah-Arizona border.

Seth Jeffs, the brother of imprisoned sect leader Warren Jeffs, told state authorities that members of the United Order of South Dakota, a trust run by the Fundamentalist Church of Jesus Christ of Latter-Day Saints, or FLDS, need more water for gardens, orchards and animal herds at its 140-acre spread about 30 miles from Mount Rushmore.

With few concrete details available, coupled with the group’s troubled history – Warren Jeffs and other church leaders have been convicted of sexually assaulting underage girls – some landowners and law enforcement officers worry that the exodus from other areas make the remote corner of the Black Hills a likely and attractive home.

“As locals, we know what is going on in there and we don’t want to see it expand,” wrote Linda Van Dyke Kilcoin, one of several neighbors who filed petitions to intervene in the group’s water request. “We value our 12-year-old girls in South Dakota.”

While Seth Jeffs pleaded guilty in 2006 to harboring his then-fugitive brother, he did not face any sex abuse charges. And South Dakota officials note that the local members of the FLDS, a radical offshoot of mainstream Mormonism, have never been in trouble with the law here. Seth Jeffs declined to comment to the Associated Press but told a local newspaper in January the compound is “not growing right now” and members “just need more water”.

The group has said about 75 people live at the South Dakota ranch, according to a 2014 drinking water report filed with the state. The fenced-in compound, complete with a guard tower, sits on hilly, rugged land the group bought about a decade ago. Valued for tax purposes at $5.3m, it’s secluded by tall pine trees along a public gravel road that cuts through it. Several buildings dot the complex, including seven large residences totaling nearly 50,000 sq feet, a chapel and school, greenhouse, and several farm buildings.

Marty Jackley, the state’s attorney general, said he’s “generally aware” of the activity at the compound and the permit request and ongoing construction at the site “are all indications that there is some level of expansion going on”. While they’ve never faced legal trouble here, Jackley said the group is on his radar for two reasons: what has gone on at FLDS sites in other states, and the additional law enforcement resources any expansion might require.

Warren Jeffs is serving a life sentence after being convicted in 2011 of abusing underage girls he considered brides. For decades, members of his church have controlled the twin cities of Hildale, Utah, and Colorado City, Arizona – but some of them are now being squeezed out by Utah authorities ordering evictions because people haven’t paid millions in overdue occupancy fees. And last year, the Texas government seized the group’s Eldorado compound on the basis that the FLDS leaders financed a $1.1m land purchase in 2003 through money laundering.

Custer County Sheriff Rick Wheeler, whose jurisdiction includes the Black Hills compound, said when the group was evicted from its Texas property, there was no indication they had moved to the compound near Pringle, citing a low level of traffic and activity. But now, he said that’s changed and the group is hauling in more gravel and lumber.

“It’s churning right now,” he said.

Both Sam Brower, a private investigator who has spent years researching the FLDS, and Willie Jessop, Warren Jeffs’s former bodyguard, agreed that leaders could be preparing to move people to South Dakota from the shrinking Utah-Arizona enclave.

“I think they are looking for places to move the elite,” Brower said, noting that Seth Jeffs, like his brother, is considered royalty among devout members.

Seth Jeffs, who originally sought to triple the amount of water the South Dakota ranch could draw, recently told the state he was scaling back his request to relieve the “fears and concerns” of the US Forest Service and the National Park Service.

William Hansen, chief of the Water Rights Branch of the NPS, said its main concern is the effect an additional well, without stricter limitations on usage, could have on Wind Cave National Park, since the FLDS draws from the same aquifer that has been forming the cave for millennia.

The group’s request for more water will likely be acted on in May. But Kilcoin, one of the concerned neighbors, doubts it’s for crop production.

“They’re not going to irrigate. You ever been to Pringle? It’s bedrock. You can’t grow anything down there. They’re bringing in a whole pile of people,” she said.