Six people were found fatally shot in a suburban Dallas home early Monday after police say two brothers made a pact to kill four family members and themselves.

Officers in Allen went to the home for a welfare check at around 1 a.m. after getting a call from a friend of one of the brothers who feared he was suicidal, said police Sgt. Jon Felty.

“It appears that the two brothers had entered into an agreement, they were going to complete suicide,” Felty said.

Felty said one of the two brothers wrote a lengthy post on social media in which he said he and his brother had a plan to kill their family members and then themselves. He also wrote that all of his decisions were based on weighing pros and cons, including the decision to kill his family.

Also read: Child among 4 dead in shooting at California office building

Police found the six family members dead in the home. Felty said the dead included the two brothers, a sister, their father and mother and a grandmother. Felty said the those dead ranged in age from 19 to mid-70s. He said the slayings likely happened over the weekend.

Felty said they were not yet releasing the names of the victims. He said the deaths were being investigated as a murder-suicide but he could not yet say who shot whom.

The brother who wrote the social media post says in it that he has been cutting himself more frequently recently and that as of late his treatment for depression — which included counseling and medication — didn’t seem to help him as much as it had previously, Felty said.

Also read: Colorado shooting: 21-year-old suspect identified

In the post, he also spent “a lot of time” writing about his disappointment with how the television series “The Office” ended, Felty said. “He thought it should have ended much differently and he was upset...”

The brother who wrote the social media post also said in it that he thought it was too easy for his brother to obtain a firearm, Felty said.

A mass killing where two siblings are the perpetrators is rare, said James Alan Fox, a criminologist at Northeastern University.

“In these situations, mass killing, it’s usually one perpetrator,” he said.

The Associated Press/USA Today/Northeastern University Mass Killing Database has recorded 452 incidents of mass killings — the slaying of four or more people — from 2006 through Sunday, he said. He said nearly half of those — 217 — were mass killings where someone killed their family members.

He said that of the 217 family mass killings, 207 involved a single perpetrator.

Also read: Atlanta-area shootings leave 8 dead, many of Asian descent

The only other family mass killing recorded in the database that involved siblings was a 2015 slaying in Oklahoma. Two brothers — Robert and Michael Bever — were sentenced to life in prison after being convicted in the stabbing deaths of their parents and three siblings. At the time of the killings, Robert was 18 and Michael was 16.

Fox said generally in killings involving two perpetrators, “usually one is the leader and the other the follower.”

“The leader feels good about the fact that someone sees them, looks up to them and is willing to do what they’re told,” Fox said. “And then the follower generally revels in the fact that the more dominant person praises them for their loyalty and strength. So it may be mutual — it may be a pact, but it’s typically one person who lays the ground rules and the other person who agrees with them.”