The two boys were found in the basement of a Daybreak home at 4781 W. Pine Canyon Lane (11730 South) shortly after gunshots were heard on the afternoon of Nov. 15.
South Jordan Sgt. Samuel Winkler said Friday there was no suicide note and that the medical examiner's determination was based solely on evidence found during the autopsies, which were performed Sunday.
"The how and why, that's the part that will take some time," Winkler said of the shootings.
Few other details have been made public other than that police found three handguns in the residence.
On Friday, police said they had learned the guns had been stolen, and that part of the case remains active.
It is believed that only one of the three guns was used in the fatal shootings, police said.
Three other boys were in the home at the time of the shootings.
Winkler said Friday he could not discuss whether the boys had been interviewed by police.
He said that four of the five boys who were in the home have "fairly minor" criminal histories, but he declined to elaborate.
Police are working closely with the Salt Lake County District Attorney's Office; however, no potential charges have been screened with prosecutors, Winkler said.
"We are just consulting the DA," he said. "We want a legal representative to make sure we are doing everything appropriately."
Winkler added that there have been no arrests.
Police have said from the beginning they were confident there were no outstanding suspects.
Police are awaiting final toxicology results from the Medical Examiner's Office, but there were no drugs or alcohol found at the scene, according to Friday's news release.