Ground and air searches turned up no sign of Sjol and were called off last week.
The family of a Williston man who disappeared under suspicious circumstances last month is offering a $30,000 reward for information that solves the case.
Jack Sjol, 58, has not been seen since April 24. Authorities say there was evidence of a violent crime at his rural home but they have not elaborated. Ground and air searches turned up no sign of Sjol and were called off last week.
"They've gone out with an airplane, they've gone out with a helicopter, we've had people on horseback, four-wheelers, searching the area and we just don't have a clue," sister-in-law Diane Sjol told KXMC-TV.
Family members are asking area landowners to report anything unusual on their property.
A similar call went out to Williston-area residents in the case of Sherry Arnold, 43, a Sidney, Mont., mother and teacher who authorities say was snatched and strangled by two men while she was out jogging in January 2012. Her body was found in a shallow grave on a farmstead near Williston two months later.
Williams County Sgt. Detective Amanda McNamee, who is spearheading the Sjol case, told The Bismarck Tribune that investigators are following up on the results of search warrants and forensic tests.
Sjol, who is not married, has lived in the area for more than 30 years. He retired from his job with the city of Williston two years ago and lives on a hobby ranch.