But while the bloodshed inside a CiCi’s Pizza and a Wal-Mart follows similar tragedies that have struck our schools, universities, shopping malls, movie theaters and beachfront towns, this latest episode is a bit of an anomaly.

This incident involved two shooters, police said, something that rarely happens in active-shooter situations like this, according to data provided by the FBI. In nearly every case — 98 percent of the time — the shooter is a sole gunman.

During and after many of these shooting incidents, police often warn that there have been reports of more than one shooter, something that almost never pans out. The reasons for this vary. People in the vicinity of a shooting calling in to report the violence may say they saw different things in different locations; if someone calls the police to report a shooting in one building, and another person reports it in a different building, the police don’t know if it’s the same person moving around or multiple shooters. In addition, if a gunman opens fire in a large area, it can take authorities time to check out every building to confirm the number of shooters.

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This has created some confusion after mass shootings, some of it lasting for several hours. The day of the Washington Navy Yard shooting last September, it took until about 10:30 p.m. — more than 14 hours after the first victim was shot — before the police confirmed that there was only one gunman. They had warned for most of the day that there could be a second shooter because it took that long to check out eyewitness accounts, D.C. Police Chief Cathy Lanier said at the time.

The two shooters in the Las Vegas violence were a man and a woman, police said, which also makes this episode unusual, because 97 percent of these shooters are male. Las Vegas police said they believe the female shooter shot and killed the male gunman before taking her own life. According to the FBI, four of 10 times, the rampages end with shooters killing themselves.

The violence also spread to two locations, with the gunfire beginning in a CiCi’s Pizza before moving to a Wal-Mart. This is less common, but it happens: One in five mass shooters will move to another location during the shooting.

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Early media reports suggested that the shooters may have been white supremacists, though this hasn’t been officially confirmed. Neighbors told the Las Vegas Review-Journal that the couple was “militant,” talked about killing police officers and would ramble about conspiracy theories. If this was the culmination of a plan, that would also fit with a pattern, as mass killers will “typically plan their assaults for days, weeks, or months,” according to a 2013 study on mass shootings by James Alan Fox and Monica J. DeLateur of Northeastern University.