A small-town Kentucky police department mourning the ambush slaying of one of its men has received threats that more officers will be targeted, prompting an investigation by the FBI and state police, the police chief said Tuesday.

The department received a written threat last week that "there are more to come," a reference to the nighttime ambush slaying of Bardstown Officer Jason Ellis, 33, last month while the K-9 officer was headed home from work, said Chief Rick McCubbin.

"We don't even know if any of the threats are credible, but obviously we're going to treat them as if they are credible," McCubbin said, adding that police would continue operating as usual, while taking extra precautions, in Bardstown, a town of 12,000 about 40 miles southeast of Louisville.

Officers are responding to calls in teams of at least two, he said, but their routines haven't been changed in light of the slaying and the threats.

"Our patrol officers are still out covering beats and making those runs," he said. "We are not going to do our job in fear. We're not going to retreat in any way."

Ellis was hit with multiple shotgun blasts on May 25 after he got out of his cruiser to pick up tree limbs strewn on a highway exit ramp in Nelson County. Authorities said they think someone purposely placed the limbs on the ramp in order to get him to stop.

Ellis was described as one of Bardstown's top officers when it came to arrests, with his bosses saying he had made a dent in the town's drug problems during his seven years with the police department.

McCubbin has said he thinks Ellis was targeted, but that the written threat didn't single out anyone in particular. The note was turned over to Kentucky State Police and the FBI, the chief said. Mary Trotman, an FBI spokeswoman in Louisville, confirmed the federal agency is investigating but wouldn't comment on specifics of the case.

A day after the written threat, McCubbin took a phone call from someone claiming to be aware of a verbal threat someone made against Bardstown police. The threat was similar, that more officers would be killed, the chief said. The matter was turned over to state police, he said.

Meanwhile, state police are still reviewing leads and interviewing people in the search for Ellis' killer, KSP Master Trooper Norman Chaffins said Tuesday. Reward money leading to the killer's arrest and conviction has surpassed $150,000, with donations from businesspeople, local governments, residents and others.

"Unfortunately, we've not received that one tip yet" that would lead to a suspect, Chaffins said.