Johnson said the investigation was ongoing Thursday, but evidence at the crash site indicated alcohol may have been a factor.
Slavens had formerly represented death-row inmate Michael Anthony Archuleta, convicted of the brutal 1988 murder of 28-year-old Gordon Ray Church a 28-year-old theater student at then-Southern Utah State College in a remote location in Millard County.
Slavens also had represented Ruben Chavez-Reyes, who was convicted of helping Roberto Miramontes Roman flee Millard County after Roman allegedly shot and killed Deputy Josie Greathouse Fox on Jan. 5, 2010.
Deputy Millard County Attorney Patrick Finlinson, who knew Slavens for 10 years, called him a very capable lawyer and one of the most experienced in the county.
Finlinson added that he could always count on Slavens putting up a good fight.
"Jim, he worked very hard for his clients," Finlinson said. "He worked hardest for the clients who liked him least."
Slavens a former Millard County prosecutor got his law degree from the University of Idaho and was admitted to the Utah State Bar in 1992.