The Ohio 17-year-old accused of America's latest school shooting tragedy chose his victims at random, his prosecutor said on Tuesday, as details of his troubled family life emerged.

T.J. Lane spoke only to confirm his birthdate and answer ‘Yes I do’ to basic questions when he appeared in a juvenile court for a preliminary hearing, hours after a third Chardon High School victim died of his wounds.

Judge Timothy Grendell imposed a gag order preventing attorneys from speaking about the case and banned the media from taking photographs of the suspect's face, while ordering Lane to be detained for a further 15 days.

"Once in custody, Lane was advised of his rights, he confessed to taking the .22 pistol to school along with a knife," prosecutor David Joyce told the juvenile court in Chardon, near Cleveland.

"He admitted he fired 10 rounds into a group of students, that he did not know the students, and that he chose them randomly."

Speaking to reporters outside, Joyce dismissed speculation about bullying and said Lane would be tried in an adult court on at least three charges of aggravated murder, provided the juvenile court judge found ‘probable cause.’

"This was the effect of one lone gunman. He chose his victims at random. This is not about bullying. This is not about drugs," the prosecutor said.

"This is about someone who's not well, and I believe in our court case we'll prove that and we'll make sure justice is done."

A picture began to emerge of a troubled boy who lived with his grandfather, his legal guardian, after his dad, who had a history of domestic violence and served time in prison, was warned by police to stay away.

Court records, pored over by the local media, showed both his father and mother had been charged with domestic violence against each other between 1995 and 1997.

The father was later charged with assaulting a police officer and served time in prison after holding a woman's head under running water and bashing it into a wall, the Cleveland Plain Dealer reported, citing court records.

Lane allegedly opened fire on Monday on a group of teenagers in the cafeteria of Chardon High School, sending terrified students running for their lives before fleeing the scene, chased by a teacher.

One victim died in hospital hours after the shooting. Two more succumbed to their wounds on Tuesday. A fourth gunshot victim remains in hospital while a fifth, the only girl shot, has been released back to her family.

Shocked residents of this close-knit community of 5,100 struggled to comprehend how the tragedy could have happened.

Crowds gathered on late Tuesday at the Church of Saint Mary in Chardon to hold a candlelit vigil after the tragic loss of three local teenagers Danny Parmertor, 16, along with Russell King and Demetrius Hewlin, both 17, their lives ended prematurely and with no explanation.

Many turned to Facebook memorial sites to express their sense of loss. "RIP Danny. Such a beautiful life cut way too short. You will never be forgotten," one post read.

As in previous school shootings, the gunman's demons, the missed warning signs and lax US gun laws leapt quickly to the fore.

The White House called the event ‘terrible and unforgivable.’

"I would very much hesitate to draw any conclusions about what happened in this incident in Ohio before there's a full investigation," spokesman Jay Carney told reporters.

Describing the mayhem, Nate Mueller, a junior at the high school, said he watched one of his friends bend over a table bleeding and another fall to the ground in a pool of blood before a shot grazed his own ear.

"It was terror," Mueller told ABC News. "There's blood, there's people screaming, everybody's just running in different directions and you're just trying to get out."

A teacher chased the shooter from the school as the town was placed on lockdown and victims were rushed to hospital, some by helicopter.

Lane was apprehended a short while later after giving himself up to bystanders.

Prosecutors have until March 1 to charge him. The next court hearing is scheduled for March 6.

The last major school shooting incident in Ohio was in Cleveland in 2007 when a 14-year-old student killed himself after wounding two teachers and two fellow students.

The deadliest school shooting in the United States was the 2007 massacre at Virginia Tech University that left 33 people dead. The worst high school shooting was in 1999 at Columbine High School in Colorado, where two students killed 12 fellow students and a teacher.