BROKEN ARROW, Oklahoma - The medical examiner has ruled on the cause and manner of death of the five members of the Bever family killed last week in their Broken Arrow home.

According to the ME, the cause of death for David and April Bever and three of their children is multiple force sharp injuries; the manner of death is a homicide.

News On 6 has also learned from sources close to the investigation that 18-year-old Robert Bever and 16-year-old Michael Bever had planned the attack on their family for some time.

Sources said the brothers wanted to kill their family first because they were considered easier targets, then, they planned to attack other people considered to be harder targets.

They also said 3,000 rounds of ammunition were delivered to the home after the murders and the boys planned to use it to kill so many people they would become more famous than the Columbine shooters.

Both Robert and Michael were booked into jail as adults and will be charged as adults to start the court case; Michael's attorney could later argue for him to be treated less harshly under the law, as either a juvenile or a youthful offender since he is 16 years old, but that is yet to be seen.

They are both being held without bond on five counts of first-degree murder and one count of assault and battery with a dangerous weapon.

Police said they stabbed their parents, David and April Bever, to death, along with their 12-year-old brother Daniel, 7-year-old brother Christopher and 5-year-old sister Victoria.

They're also accused of trying to kill their 13-year-old sister who survived. They also had a 2-year-old sister who was not hurt.

Police said it was 12-year-old Daniel who called 911 and told police his brothers were attacking the family before he passed away. The 13-year-old survivor confirmed for officers her brothers were responsible.

Officers followed the brothers’ footprints from the back door to a wooded area behind the home where they were captured.

Tulsa County Jail records show Robert is scheduled to appear in court August 3.