"There was no indication of a struggle" between mother and son, Sedensky said.
From there, he drove to the elementary school -- leaving a 12-gauge shotgun in the car, police would discover -- and shot through the front entrance.
The information detailed Thursday dispelled some earlier reports on what happened, Sedensky noted. Lanza wasn't wearing a bulletproof vest, for one. And besides the hallway, he fatally shot students and teachers in two classrooms, not three.
When police found Lanza's body -- killed by a single, self-inflicted shot from a Glock 10 mm handgun -- they also discovered that the Bushmaster rifle was loaded with 14 bullets in its 30-round capacity magazine, plus one round in a chamber.
This was one of 10 of this firearm's 30-round capacity magazines at the scene, Sedensky explained. More ammunition for the Glock and a Sig Sauer P226 9 mm handgun was also found. Three such magazines still contained 30 rounds. There were six more magazines nearby -- three of them were empty, while the others had 10, 11 or 13 live rounds in them.
"One-hundred-and-fifty-four spent .223 casings were recovered from the scene," the state's attorney wrote, indicating that Lanza had fired at least that many bullets from what Connecticut authorities had described as an "assault-type rifle."
A lot of bullets fired in not a lot of time. Sedensky said, "It is currently estimated that the time from when the shooter shot his way into the school until he took his own life was less than five minutes."
Gun control push continues despite dwindling support
The full investigation into the Sandy Hook shooting, Sedensky said, likely will be completed this summer.
After that happens, the state's attorney said he will write a report evaluating the crimes committed and whether or not anyone will be prosecuted as a result.
Until then, the debate over gun control stoked by the Sandy Hook massacre continues in earnest.
Recent polling shows public support for gun restrictions has declined since the shooting. Shortly after it, 52% of Americans favored major restrictions on guns, according to a CNN/ORC International poll. That number has dwindled to 43%, a version of the same poll conducted between March 15 and 17 found.
No legislation on the matter has passed yet in Washington, though gun control advocates are pushing.
Mayors Against Illegal Guns on Thursday released its first television ad featuring relatives of Newtown victims urging politicians to pass tougher gun laws. New York Mayor Michael Bloomberg, the group's co-founder, said in a statement: "We cannot afford to wait for another tragedy. It's long past time for elected officials to listen to their constituents and pass reforms like comprehensive background checks that we know will save lives."
President Barack Obama voiced a similar sentiment in remarks Thursday as part of a "national day of action" to galvanize supporters pushing for tougher gun laws.
"This is our best chance in more than a decade to take common sense steps that will save lives," he said at a White House event. "If there is a step we can take that will save just one child, just one parent, just another town from experiencing the same grief that some of the moms and dads who are here have endured, then we should be doing it. We have an obligation to try."