(CNN) -- Authorities had three previous encounters with Jerad and Amanda Miller this year, including one instance days before the couple went on a shooting rampage in Las Vegas.
The couple killed two Las Vegas police officers, Alyn Beck, 41, and Igor Soldo, 31, at a pizza restaurant Sunday.
Shortly after, they fatally shot shopper Joseph Wilcox, 31, at a local Walmart.
As more details emerge, assistant Sheriff Kevin McMahill said authorities had previous contact with the two shooters in February, April and May -- all without incident.
The anti-police sentiment the couple expressed when they gunned down two officers was not evident at previous meetings, he said.
The first meeting was in February, when detectives investigated Jerad Miller's threats against the Indiana Bureau of Motor Vehicles, McMahill said.
Miller threatened to shoot staff at the agency after his suspended driver's license was seized during a traffic stop, according to Danielle Dean, a spokeswoman for the Indiana Bureau of Motor Vehicles.
Nevada detectives who interviewed Miller about the Indiana incident did not see any signs of potential threats, according to McMahill.
A second contact came on April 10.
At the time, officers interviewed the Millers as witnesses in a domestic violence case, McMahill said.
They provided voluntary statements about the alleged assault and did not display any "anti-police" sentiments, he said.
On May 31, the Millers again provided statements to police officers about an alleged sexual assault of a neighbor. In that case, their meetings were without incident as well.
"Because an individual may online espouse ideology that is anti-government or anti-police does not make them translate into a murderer," McMahill said. "What happened to change these two people into murderers, we don't know."
The killers apparently considered law enforcement as oppressors, authorities said.
When they killed the officers over the weekend, they placed a "Don't Tread on Me" flag and a Nazi swastika on one officer and left a note saying, "This is the beginning of the revolution," on the other officer's body.
They then ran to a nearby Walmart, where they killed Wilcox before barricading themselves inside the store during a firefight with officers.
Sheriff's deputies fired the bullets that killed Jerad Miller, a sheriff's department official said Wednesday. His wife shot herself.
Authorities had earlier said his wife killed him before taking her own life. But McMahill told reporters Wednesday that a review of forensic evidence and autopsy results showed that police fatally wounded the gunman.
"We do not believe any of her shots hit him," he said.
McMahill said investigators were reviewing graphic video of the rampage.
"The video shows you exactly how vicious and coldblooded these murderers were," he said.
A Walmart security video released by police showed the Millers in their final moments.
In the video, they lay wounded in an aisle surrounded by items they used for cover. They are seen holding firearms, which they at times point at each other.
At the end of the video, Amanda Miller turned the gun toward herself. She was not shown firing it.
CNN's Kyung Lah reported from Las Vegas, and Faith Karimi wrote and reported from Atlanta.