A 35-year-old father of four and his 24-year-old brother have gone missing after traveling from their homes in Wisconsin to Missouri on business.

Justin and Nick Diemel, 24 and 35, respectively, who own Diemel Livestock in northeast Wisconsin, were supposed to come home Sunday but didn't make their scheduled flight, officials said.

Missing brothers Justin, left, and Nicholas Diemel. Clinton County Missouri Sheriff's Office

Their rental truck was found in a commuter parking lot in Holt, about 30 miles northeast of the Kansas International Airport, on Monday with no sign of the two men, authorities said.

Nick Diemel's wife said she spoke to her husband on the phone Sunday morning before they were supposed to visit a client.

“Nick is a father of four kids. He's my high school sweetheart,” Lisa Diemel told WMTV, am NBC affiliate in Madison, Wisconsin.

"Him not contacting us... we didn't go for long when he was on trips to check on me or talk to our children."

The Clinton County Sheriff’s Office said Wednesday that searches "have been conducted and are underway in Clinton and Caldwell counties" and that "multiple agencies are involved in the investigation, and multiple leads continue to be researched."

Family members in Wisconsin expressed shock about the brothers' disappearance.

"I have no clue what's going on,” Nicholas' and Justin's mother, Pam, told WGBA, an NBC affiliate in Green Bay, Wisconsin.

“I can't even believe that something like this can happen. You go on a business trip and you're coming back with the person you met with. They didn't make their flight. They found the abandoned truck on the side of the road with their carry-ons and that was it."