The alleged killing by two teenage brothers of their parents and three siblings, ranging in ages from 5 to 12 years old, in their Broken Arrow, Oklahoma home on Wednesday night became the fifth incident of mass killings in just one week in the United States that left 23 persons dead, becoming "one of the deadliest stretches of homicides that include at least four victims in several years," reports said.

The list of mass killings does not even include the shooting in a movie theatre in Lafayette, Louisiana, on Thursday night, which resulted in the death of three people, including the gunman who apparently killed himself after the shooting. Based on FBI definition, the killing of just two people does not qualify as a mass killing.

Since July 15, five mass killings have occurred in the US with 23 people killed. Besides the Broken Arrow case, five people including three kids were killed in Modesto, California (July 18); a mother and her three kids were found dead in Chicago (July 18); five were killed by a gunman in Chattanooga, Tennessee (July 16); and four persons were killed in Holly Hill, South Carolina (July 15).

Mass killings, according to the FBI report, happen about every two weeks and have a "contagion" effect.

"While only [constituting] about 1 percent of all murders nationally, mass killings still happen frequently," the report said. "The FBI counted 172 cases of mass killings between 2006 and 2011. That does not include some large states such as Florida, for example. Poor reporting by police agencies to the FBI also means some mass killings were left out, while others that don't meet the standard were included."

It added that nearly three in four of the guns used in mass killings were handguns and one-third of mass killers don't leave the crime scene alive—either they kill themselves or are killed by the police.

"I don't think people are aware of just how common mass killings are, that they actually happen every 11 to 12 days," USA Today's Jodi Upton told NBC News. "It's pretty unusual to see five in 10 days."

Since 2006, nearly 1,400 victims have been killed in 284 cases of mass killings and 94 percent of the suspects are male.

"There's very few women who commit mass killings. But of those who do, there's a slight tendency to prefer drownings or strangulation or arson," said Upton.

She said more than half of mass killings occur among family.