The mayor of Las Vegas has praised two police officers shot dead in an ambush while eating lunch for dedicating "their lives to protecting all of us in our community".
The shooters, a man and a woman, killed the officers at point-blank range before fleeing to a nearby store, where they killed a third person and then died in an apparent suicide pact, authorities said.
One of the shooters yelled, "This is a revolution", but the motive for the killings remained under investigation, a Las Vegas police spokesman said. He added that "we don't know anything about the suspects yet".
Mayor Carolyn Goodman called the killings a "cruel act".
She said: "What precipitated this event we do not know. My officers were simply having lunch when the shooting started.
"It's a tragic day, a very difficult day. But we still have a community to police, and we still have a community to protect. We will be out there doing it with our heads held high but an emptiness in our hearts."
Nevada state governor Brian Sandoval said he was devastated by the murders of the two officers and an innocent bystander in an "act of senseless violence".
The shooters walked into CiCi's Pizza on Sunday and gunned down officers Alyn Beck, 41, and Igor Soldo, 31, police said.
One of the officers was able to fire back before he died, but it was unclear if he hit the suspects, sheriff Doug Gillespie of the Las Vegas Metro Police said.
After shooting the officers, the suspects fled to the Wal-Mart across the street, where they killed a person inside the front door. The victim's identity hasn't been confirmed, although media said it was a woman.
The suspects then exchanged gunfire with police before The female suspect shot the male suspect, then took her own life, police said. The suspects' names were not released.
A police spokeswoman, Laura Meltzer, said the two suspects died after they "engaged in what is being described as a suicide pact".
Both officers were pronounced dead at University Medical Centre. Beck had been with the department since 2001 and leaves behind a wife and three children, while Soldo had been with the force since 2006 and is survived by a wife and baby.
Pauline Pacheco was at Wal-Mart when she saw the armed man and grabbed her father to escape, KLAS-TV reported.
"We saw when the man was walking, he was shouting, yelling bad words, and suddenly he had a gun," she said. "It was terrible … That man was crazy."
Assistant Sheriff Kevin McMahill said the male suspect yelled "everyone get out" before shooting towards the Wal-Mart. The suspects then walked to the back of the store.
Wal-Mart expressed its condolences and said the company was working with police on the investigation. CiCi's Pizza said the company was deeply saddened by the shooting and would keep the location closed until further notice.
Additional reporting by Reuters