Chaffins said investigators determined the shooting was an ambush. He said anyone who may have seen something suspicious in the days leading up to the shooting is asked to contact Kentucky State Police.
"Because it was premeditated, we believe someone was in that area before," Chaffins said. "If anyone saw a car parked that wasn't broke down, or people milling around, just anything, we're asking them to call us."
He said investigators are trying to determine if Ellis was the target, "or whether it was the first person to pick up the debris."
Chaffins said authorities have received at least two tips. A reward is now up to $6,000.
"We've had a couple of people call in," he said. "We know there was another motorist that went around the debris in the roadway prior to [the shooting]. But we have no tips that have led to suspects at this time."
The shooting happened near Bardstown, a city of about 12,000 people about 40 miles southeast of Louisville.
Mark Mays is a city councilman in Bloomfield, about two miles from the location of the shooting. He told The Courier-Journal that many residents are praying for Ellis' family, but are "concerned about what happened this close to our community."
"There's not a lot of types of crime like this around here," Mays said. "Small-time drug use, breaking and entering, vandalism maybe. But something like this? No."
Ellis is survived by a wife and two children. Funeral services are scheduled for this Wednesday and Thursday in Bardstown.
According to the University of the Cumberlands website, Ellis was a star baseball player when he attended the school and signed with the Cincinnati Reds minor league program, playing from 2002 to 2005.
The school's biography says that since becoming an officer, Ellis has twice received the Governor's Award for Impaired Driving Enforcement and also was awarded Officer of the Year in 2008.