"Unfortunately, the only two that know exactly what happened are deceased," Maglish said.
Detectives are waiting on ballistic evidence in an effort to determine just what happened Saturday night inside the home at 859 W. Potomac.
Police said no one else was involved in the teens' deaths.
Maglish said the two boys were best friends and lived across the street from each other.
"This is two families grieving as one," he said. "Because these boys were just so close together."
The two boys were spending Saturday evening together when Nelson's mother and stepfather Tammy and Jack Bridwell went out for no more than an hour Saturday night, leaving both boys home alone. When they came back about 7:25 p.m., they found both boys lying shot in different parts of the house. Each was the victim of a single gunshot wound from the same handgun, which is owned by one of Nelson's parents.
"Obviously they came home to a horrific scene," Maglish said.
No one else was home when the boys died.
Nelson's parents tried fruitlessly to revive the teens, but Peck was already dead. Nelson was transported to an area hospital where he died Sunday night.
With Nelson's death, the investigation Monday had shifted to see if police can piece together the last moments of the boy's lives.
He said police are exploring whether the shooting might have been a murder-suicide, a suicide pact or possibly an accidental shooting that turned into something far worse.
But since neither boy can tell police what happened in those final moments, they're relying on ballistics and science for the answer, Maglish said. Those results will likely take weeks or months to come back.
Speaking to media from the steps of the Bridwell home Monday afternoon, the Bridwell family's attorney Douglas Thayer said that losing a child is incredibly difficult."To not know why adds a whole other layer of stress," Thayer said, "of wondering for the rest of your life why it happened."
Nelson was well-loved by his parents, Thayer said, including his father Gary Nelson in that statement.
"Andrew liked music, skateboarding" and most recently had developed an interest in photography, Thayer said, adding that the two boys were really good friends and had been for many years.
Isaac Dunford, 19, lives directly east of the Bridwells and said he spent time recently with the family.
"They just got Andrew a puppy," Dunford said of the little chocolate Labrador the Bridwells acquired just a few days earlier.
"First they named her Sandy, but after this incident they renamed her Angel because they believe it was Heavenly Father's way of helping them get through this," Dunford said.
At the nearby home where Ashton Peck lived with his mother and three sisters, a makeshift memorial had been erected with a large blue bear, flowers and an enlarged Facebook photo of the two teens posted in a window.
Tiffiny Lee has lived just west of Ashton Peck for four years and described him as "a nice boy, a sweet kid who loved his young cousins."
"It's heartbreaking," Lee said of Sunday's tragedy.
The boys attended school at Murray High School and Riverview Junior High School.
Sitting outside the high school Monday afternoon, Alyssa Lofgren, 15, said she and Nelson had been pretty good friends 7th through 9th grades at Riverview Junior High. "He really liked hanging out with friends, listening to music, doing his own activities," Alyssa said."He seemed so happy last time I saw him" which she said was the first day of school, Aug. 26, since the two did not share any classes together.
The district said counselors were being provided Monday for students and staff. Counselors were also on hand at Grace Elementary School, where both boys had attended previously.