Police are working to make sense of the terrible tragedy that struck Newtown, Conn., but so far they have been tight-lipped about why they think Adam Lanza killed 27 people, including 20 young children, before turning a gun on himself.

Since the deadly shooting unfolded Friday, police have held regular press conferences to apprise the media of what they know.

But in the barrage of reporters' questions concerning what exactly happened in the school -- and what might have led Lanza to do it -- officials have refused to disclose many details.

The investigators' communications pointman, Conn. State Police Lt. Paul Vance said without elaboration Saturday they had found "very good evidence ... that our investigators will be able to use in painting the complete picture, the how and, more importantly, the why."

When pressed, he did not describe the nature of the evidence.

Vance was no more forthcoming on Sunday, when he made clear his cautious approach would continue.

"I cannot restate or state heavily enough: I have not and will not put out a timeline in this criminal investigation as it is underway," Vance said. "It is inappropriate, we will not do that."

He acknowledged the intense interest in what compelled Lanza, but said the public should not expect more than a trickle of information as the investigation continues.

"All information relative to this case is coming from these microphones. And any information coming from other sources cannot be confirmed and, in many cases, it has been found, is inaccurate," he said.

The police have confirmed that Lanza was armed with two semiautomatic handguns, a high-powered rifle and hundreds of rounds of ammunition in high-capacity clips when he forced his way into the Sandy Hook Elementary School.

The chief medical examiner said the ammunition was the type designed to inflict maximum amount of bone and tissue damage by exploding after impact, and that each of the victims were shot multiple times with the rifle. Some were struck from close range.

Authorities said Lanza had no criminal record, but have not confirmed the accounts of school officials and staff describing the gunman as a withdrawn young man with behavioural issues and a mild form of autism.

Nor have they confirmed reports Lanza was once a student at the Sandy Hook School, or that his mother had past ties there.

Vance said Monday that investigators hope to paint a complete picture of the shooter's life, by the time their work is done.

"We will go back to the date of his birth," Vance said.

Lanza lived with his mother Nancy, a 52-year-old divorcee. Investigators said she owned the guns Lanza used, and that they had not found any evidence Lanza had honed his shooting skills ahead of the attack.

Watching the developments from Washington, former FBI Special Agent Brad Garrett says, based on the scant information so far, he sees parallels between the Newtown massacre and similar recent mass shootings.

Historically, he said, the perpetrators of such crimes are found to have been socially isolated individuals who've experienced difficulty in school, for example, compounded by dysfunction at home.

Feelings of inadequacy build as these typically male individuals reach their late teens or early 20s.

Garrett told CTV's Canada AM, that's when something snaps.

"They basically decide that they are so powerless and no one's going to help them and that we, society, have wronged them -- or in some cases particular individuals have wronged them -- and the only way to gain back a feeling of power and control is to inflict pain."

For those wondering how anyone could choose to exact that suffering on Grade 1 children, Garrett said someone in that mindset might consider their victims as targets rather than human beings.

Police have promised to reveal whatever they learn about Lanza's motivation, but not before their painstaking investigation is wrapped up.

"Again, motive, as I discussed earlier -- that will come as we finish the investigation," Vance cautioned over the weekend. "We simply can't piecemeal it, we don’t have a specific reason that we can stand here and say that this occurred."

In U.S. history, the Newtown massacre now stands as the second-deadliest school shooting after the 2007 shootings at Virginia Tech University, in which 33 people including the shooter were killed.

With files from The Associated Press