(CNN) -- Police scouring the scenes where a couple shouting "revolution" gunned down two police officers and a civilian before killing themselves have found a possible "manifesto" among their belongings, a law enforcement official told CNN on Monday.
The official didn't disclose what's in the writings found at one of the scenes, and authorities have yet to publicly discuss a motive for Sunday's shootings at a Las Vegas pizza restaurant and outside a Walmart store.
But a police source told CNN on Sunday that the couple held extremist views toward law enforcement.
In addition to the writings, investigators also found flags or insignias with markings on them and two backpacks at the scene. It's not known what, if anything, was found in the backpacks, the source said.
Investigators searched an apartment believed to be where the couple lived late Sunday night. They also are going through the couple's social media postings, the official said.
While authorities have yet to identify the couple, a woman who says she lived near them told CNN affiliate KTNV that they liked to dress up as the villainous Batman characters Joker and Harley Quinn.
The neighbor, Krista Koch, told the station the man also sometimes dressed as Slenderman, a fictional horror character that recently surfaced in the recent stabbing of a 12-year-old girl in Wisconsin.
Koch also told the station the couple had told her they were going to carry out an attack, but she thought they were "crazy," so she dismissed what they said.
Police say the couple shot Officers Alyn Beck and Igor Soldo as the two ate lunch at CiCi's Pizza.
Witnesses told police the shooters said, "This is a revolution" during their attack.
They then ran across a parking lot to the Walmart, where they shot a woman before retreating to the back of the store, where the woman fatally shot her husband before killing herself.
"This was a senseless and cruel act killing three innocent people," Las Vegas Mayor Carolyn Goodman said. "Two who dedicated their lives to protecting all of us in our community and one who was innocently going about her daily life."
'Simply having lunch'
The pair began their attack about 11:20 a.m. Sunday, when they opened fire inside the restaurant where Beck, 41, and Soldo, 31, were having lunch.
"I just sat down to have lunch. The officers were sitting in front of me, at the table right next to me, and this man came in out of nowhere," a dazed Sheree Burns told KTNV.
"I thought he was going to get a drink, the way he walked up and walked past them. And then he turned around, pulled a gun on his right and shot the bald officer in front of me."
Sheriff Doug Gillespie said one officer was able to return fire.
"What precipitated this event, we do not know," he said. "My officers were simply having lunch."
The suspects took the officers' guns and ammunition, Assistant Sheriff Kevin MacMahill told reporters. They "made a statement -- something to the effect of, 'This is a revolution.' "
'That man was crazy'
As they walked out, they passed Alvaro Lopez.
"They had a backpack, and I saw a gun in their hand," Lopez told CNN affiliate KLAS. "He just told me to tell the cops that it was a revolution and that he'd just killed two cops inside CiCi's."
The pair ran to the Walmart across the parking lot. At the store's entrance, they killed a woman. Her name has not yet been released.
Shoppers rushed toward the exit.
Pauline Pacheco saw the armed man and grabbed her father.
"We saw when the man was walking," she told KLAS. "He was shouting, yelling bad words, and suddenly he had a gun.
"It was terrible, it was terrible. That man was crazy."
Officers converged and exchanged fire with the couple, who ended the brief exchange in what MacMahill referred to as a "suicide pact."
'Emptiness in our heart'
Beck and Soldo were both married with children: Beck left behind a wife and three children; Soldo, a wife and a baby.
A prayer vigil is planned Monday night, KTNV reported.
Meanwhile, Las Vegas police will be doubling up in patrol cars for at least the next few days as officers mourn the loss of two colleagues.
"We still have a community to police and we still have a community to protect," Gillespie said Sunday night. "We will be out there doing it with our heads held high but with an emptiness in our heart."
CNN's Jack Hannah reported from Las Vegas; CNN's Shimon Prokupecz; Matthew Stucker and Janet DiGiacomo contributed to this report.
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