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By Elizabeth Chuck

Two Oklahoma teenagers have been formally charged with murdering five family members following a horrific attack in their home in an upscale suburb of Tulsa.

Robert Bever, 18, and his 16-year-old brother were arrested after their parents and three of their siblings were found stabbed to death in Broken Arrow on July 22. The teens were officially charged on Friday as adults, reported NBC affiliate KFOR.

Robert BeverBroken Arrow Police Department

NBC News is not identifying the 16-year-old because he's a minor.

Authorities arrived at the house to find a gruesome scene: Faint cries for help led them to a 13-year-old sister of the boys who was stabbed several times, critically injured but alive. The parents, David and April Bever, and three children who were between the ages of 5 and 12, were dead.

“I was told knives, there was a hatchet, there was several different types of weapons that were actually used in the homicides,” said Cpl. Leon Calhoun of the Broken Arrow Police Department, according to KFOR.

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Broken Arrow police have said the 13-year-old told them her brothers were responsible for the attack. A 2-year-old was also found in the house, unscathed.

Bloody footprints eventually led authorities to the two brothers, who allegedly fled out the back door when officers arrived.

“I don’t know if there was any remorse or anything going on at this point,” Calhoun added.

RELATED: Neighbors Stunned by 'Gruesome' Stabbing Massacre

The two are charged with five first-degree murder counts and with assault and battery with intent to kill for the attack on their surviving sister.

Broken Arrow, a city of about 100,000 residents, is about 15 miles outside of Tulsa.