(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.

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