Two police officers were "simply having lunch" at a strip mall pizza buffet in Las Vegas when a man and a woman fatally shot them in point-blank ambush, then fled to a nearby Walmart where they killed a third person and then themselves in an apparent suicide pact, authorities said.

The attack at a CiCi's Pizza restaurant Sunday killed Officers Alyn Beck, 41, and Igor Soldo, 31, who are both husbands and fathers. One of the shooters yelled, "This is a revolution," but a motive remains under investigation, Las Vegas police spokesman Larry Hadfield said.

"It's a tragic day," Sheriff Doug Gillespie said at a news conference Sunday afternoon. "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 with an emptiness in our hearts."

Photos of slain Las Vegas police officers Alyn Beck and Igor Soldo are displayed during a news conference Sunday afternoon. (Steve Marcus/Reuters, Las Vegas Sun)

For added safety, officers who normally work alone will be paired up with another officer for a time, Gillespie said.

The deadly rampage in the aging shopping centre about five miles northeast of the Las Vegas Strip took place in a matter of minutes. Police were called at 11:22 a.m. to the pizzeria, where one of the officers was able to fire back at his assailants. It's unclear whether he hit them, Gillespie said.

Shots were reported five minutes later at a nearby Wal-Mart, where the shooters gunned down a person just inside the front door and exchanged gunfire with police before killing themselves, police said.

The female suspect shot the male suspect before killing herself, Gillespie said. The victim's identity hasn't been confirmed, and the suspects' names haven't been released.

'Suddenly he had a gun'

Both officers were pronounced dead at University Medical Center. Beck had been with the department since 2001 and leaves behind a wife and three children. Soldo had been with the force since 2006 and is survived by a wife and baby, police said.

He was described as a good father and a "great guy" by his sister-in-law, Colleen Soldo of Beatrice, Nebraska. She said he attended high school in Lincoln, Nebraska, and previously worked as a corrections officer.

Sheree Burns, 48, told the Las Vegas Sun she was eating at the restaurant, seated just behind the two officers.

A man came up to one of the officers and shot him in the head, Burns said. She said she ducked under her table but peeked up and saw the other officer being shot.

She said the man took an officer's handgun and the two attackers fled.

Police say an armed couple ran into a Wal-Mart store after shooting two police officers at a nearby pizzeria and killed a man who confronted them before killing themselves in an apparent suicide pact. (John Locher/Associated Press)

Pauline Pacheco was shopping 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 told the station. "It was terrible, it was terrible. That man was crazy."

Assistant Sheriff Kevin McMahill said the male suspect yelled "everyone get out" before shooting at Wal-Mart. The suspects then walked to the back of the store.

'Senseless act of violence'

Gov. Brian Sandoval said in a statement he was devastated by the murders of the two officers and an innocent bystander in an "act of senseless violence."

In a statement, Las Vegas Mayor Carolyn Goodman called the killings a "cruel act" and praised the officers for dedicating "their lives to protecting all of us in our community."

Wal-Mart employees and shoppers were taken to a nearby women's clothing store to be interviewed by police. The restaurant and Wal-Mart remained closed as detectives processed evidence. McMahill said the investigation is "very complex" because it involves more than 1,000 witnesses.

Wal-Mart expressed its condolences in a statement and that the company is working with police on the investigation. Cici's Pizza said in a statement the company was deeply saddened by the shooting and would keep the location closed until further notice.

Sunday's killings come less than a year after the Las Vegas police department's most recent on-duty death. Officer David VanBuskirk died while rescuing a stranded hiker by helicopter on July 22, 2013.

The department has lost officers over the past decade in vehicle accidents and in an off-duty shooting, but the most recent on-duty shooting death happened Feb. 1, 2006, when Sgt. Henry Prendes was ambushed during a domestic violence call.