A 10-year-old boy woke up to find his mom, dad and three older brothers shot to death in what police believe to be a murder-suicide in Oklahoma City, authorities say.

The Oklahoma City Police Department said they believe the boy's father "systematically" went through the house between Sunday night and Monday morning shooting his sons after he shot his wife multiple times.

"Those children were hunted down and killed," said Master Sgt. Gary Knight, the assistant public information officer for the Oklahoma City Police Department, at a press conference Tuesday morning.

However, police say that the father left his youngest son, the 10-year-old who later called the police, unharmed.

"We don't know why he was left unharmed," said Knight. "I know that's the question everybody has, that's the question we have, too. In the end, nobody's ever gonna know the answer to that."

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Who was killed?

According to police, the following were killed in the shooting:

• 39-year-old female- Police say she was the mother of the children and wife of the suspect.
• 42-year-old male- Suspect who police say died from a self-inflicted gunshot wound after he shot his wife and three oldest sons.
• 18-year-old male- The oldest son of the couple. He would've turned 19 next month.
• 14-year-old male- Son of the couple who was also shot.
• 12-year-old male- Son of the couple and youngest murder victim. He just turned 12 in March.

What happened?

Around 9:35 a.m. Monday morning, the child called police and alerted them to the incident at the house near Yukon, roughly 20 miles west of downtown Oklahoma City.

When police arrived, Knight said they found the 10-year-old outside before they went into the house, where they found the five bodies that belonged to the people listed above.

After confirming they could not save anyone in the house, Knight said they left and waited for a warrant so they could process the scene.

Knight adds that the boy was placed in the custody of a relative, but did not specify who.

According to Knight, investigators determined that the suspected shooter got into an altercation with his wife. It ended when he armed himself with a gun and shot her multiple times.

Police say victims were found in different parts of the home

He then went through the house and killed his sons. The motive for the killings is unknown, said Knight. He added that the bodies were found in different areas of the two-story house.

One of the male victims and the female victim were found downstairs, while two of the male victims were found upstairs. At least one of the male victims was shot in bed, Knight told USA TODAY. But, aside from the female victim, he is not sure who was shot where.

Knight told USA TODAY that police believe the suspected shooter began his rampage downstairs, then went upstairs before returning to the first floor and killing himself.

Family was flying 'under the radar', Sgt. Knight says

At the press conference, Knight added that there was no known history of domestic problems between the couple, but said police are going to speak with friends and family to see if they know of anything else about their relationship.

He told USA TODAY that police had no calls or reports of any domestic violence from the family.

"We had no indication that anything like this was going to happen," said Knight.

He said usually when people "annihilate" their families like this, there is a history of domestic violence reports and protection orders, but he said this family was "really flying under the radar."