The account's profile name was in Russian and translated to the surname of the attacker who killed 26 people at Sandy Hook Elementary School in Connecticut in 2012.
A police incident report describing last year's interview with the boy and his father was released on Thursday.
In the report, a deputy described the boy as "reserved" and "calm" and said he "assured me he never made any threats to shoot up any school".
Police records reveal that the boy's mother and father were in the process of divorcing, and he was staying with his father during the split.
The teen often hunted with his father, who told police he had photographed his son with a deer’s blood on his cheeks.
The boy's maternal grandfather told the New York Times he partly blames the tumultuous home life after Mr Gray's split from his daughter.
“I understand my grandson did a horrendous thing - there’s no question about it, and he’s going to pay the price for it,” Charlie Polhamus told the newspaper.
"My grandson did what he did because of the environment that he lived in,” he added.
During the news conference on Thursday, Barrow County Sheriff Jud Smith said all nine of those injured were expected to make a full recovery.
Several victims had already left hospital, he said.
Students Mason Schermerhorn and Christian Angulo, both 14, and teachers Richard Aspinwall, 39, and Christina Irimie, 53, died in the attack.
Witnesses said the suspect left an algebra lesson on Wednesday morning only to return later and try to re-enter the classroom.
Some students went to open the locked door, but apparently saw the weapon and backed away.
Witnesses said they then heard a barrage of 10-15 gunshots. Two school police officers quickly challenged the boy and he immediately surrendered.
These are not the first charges against the parents of a suspect in a school shooting.
In April, the parents of a Michigan teenager who killed four students with a gun they bought for him just days before the shooting were sentenced for their role in the attack.
James and Jennifer Crumbley were both found guilty of manslaughter and each sentenced to 10 to 15 years in prison.
The case was widely reported to be the first time the parents of a child who had carried out a mass shooting were held criminally liable.
