Washington, Feb 17 (IANS): Nikolas Cruz, the 19-year-old youth who killed 15 students and two staff members with an AR-15 automatic rifle in Florida, told police that voices in his head told him how to carry out the attack, police said on Friday.

Cruz was arrested and charged on Thursday on 17 counts of premeditated murder, after confessing to the mass shooting of students at Marjory Stoneman Douglas High School in Parkland on February 14, Valentine's Day, Efe reported.

The former student at the same high school remains in custody without the possibility of parole.

The voices Cruz heard were described as "demons" by a police source on the ABC News channel.

As time passes, new information comes to light about Cruz's life before he opened fire with an assault rifle to gun down students at his former high school, from which he was expelled for disciplinary reasons.

The rifle used by Cruz was legally purchased by the youth a year ago at Sunrise Tactical Supply, an arms store at a Coral Springs shopping mall in South Florida.

Douglas Rudman, the attorney representing the store's owners, Michael and Lisa Morrison, said that "Cruz purchased the assault rifle off the rack, in a box, and did not purchase any ammunition with it other than the magazine that came with the gun".

"It is for that reason that no red flags were raised," Rudman said.

"It seems like Cruz made a deliberate attempt to not draw suspicion by not buying too many things from one place," the attorney said.

Authorities confirmed that the gun was purchased legally.

The owners of the gun store had completed the entire security protocol required by the Justice Department, which included obtaining a copy of Cruz's driver's license and having him answer a questionnaire about any mental illness he might have.

"What I understand is that was answered appropriately," said Stuart Kaplan, an attorney who also represents Sunrise Tactical Supply.