Investigators looking for the killer of a Bardstown police officer are asking Nelson County-area residents to contact them if they had tree work done around the time of the ambush.Officer Jason Ellis was shot to death during the early morning hours of May 25 after he got out of his cruiser to pick up debris on a highway exit ramp in Nelson County.Police won't describe the debris on the ramp off the Bluegrass Parkway.Kentucky State Police Master Trooper Norman Chaffins said that Nelson County-area residents are asked to contact police if they had trees trimmed or removed from their property around the time of Ellis' slaying.Chaffins says information indicates that tree trimming might be linked to the murder.He says police have no suspects, and the reward is up to $125,000. That total is expected to go higher.Restauranteur Jeff Ruby reportedly plans to add $33,000 to the reward fund.