CNN  — 

Investigators looking for two missing Wisconsin farmers found human remains Tuesday on the Missouri farm they’ve been searching.

The search for Nicholas Diemel, 35, and his brother, Justin Diemel, 24, turned into a death investigation last week after the brothers had been missing for several days.

“This is still an active investigation, and human remains have not been identified at this time,” Clinton County Sheriff Larry Fish said at a brief press conference. The causes of death haven’t been determined, Fish said.

The Diemels’ rented truck was found abandoned in a commuter lot last week, and authorities arrested Garland “Joey” Nelson after he admitted driving the brothers’ truck from his farm to a commuter parking lot and leaving it there.

Nelson, who owns the farm the brothers visited. has been charged with tampering with a vehicle and is being held without bond, CNN affiliate WBAY reported. Fish said he is still the only person who’s been arrested in the case.

The brothers, owners of a livestock company, were in Missouri for a cattle deal. Their company has contracts with farmers in multiple states to raise cattle for them, CNN affiliate KCTV reported. Nelson’s Braymer, Missouri, farm was one of several farms they were going to visit last week, the affiliate said.

“Investigators are still working with Frontier Forensic doctors to determine what took place,” Fish said. The sheriff declined to give any further information.

Nelson has been the subject of previous investigations, including a federal one, Fish said. In 2015, when Nelson was 21 years old, he pleaded guilty to a cattle fraud scheme that resulted in losses of nearly $300,000 for the victims, according to a news release from the US Attorney’s Office in the Western District of Missouri.

Nelson’s three-part fraud scheme included loan fraud, livestock sales and insurance fraud, the attorney’s office said.

A year later, Nelson was sentenced to two years in federal prison without parole and ordered to pay $262,450 to the victims.

CNN’s Christina Maxouris contributed to this report.