Lanza then went to the elementary school, shooting his way inside and opening fire on classrooms with a semiautomatic rifle before killing himself to end the rampage, which lasted less than five minutes, the documents showed.
"This is exactly why we need to ban high-capacity magazines and why we need to tighten our assault weapons ban," Connecticut Gov. Dannel Malloy said Thursday. "I don't know what more we can need to know before we take decisive action to prevent gun violence."
After the Newtown shootings, some states -- including New York -- passed tougher gun laws.
While the fervor for stronger legislation was high in the immediate aftermath of the shootings, a CBS News survey released this week indicates a 10-point decrease in support of stricter gun laws, from 57% immediately after the Newtown shootings to 47% now.
That poll was in line with a CNN/ORC International survey released last week that indicated a 9-point drop in the percentage of Americans who favor major restrictions on guns or an outright ban on gun ownership, from 52% following the shootings to 43%.
Other polls have shown changes in the same downward direction.
"Opinion on gun control was fairly steady over the past few years, but seemed to spike after the Connecticut shootings," CNN Polling Director Keating Holland said. "The big question is whether support for major new gun laws has simply dropped back down to that previous level or whether the slide will continue even further."
He noted that the biggest drop came among two specific demographics -- older Americans and people who live in rural areas.
"In the immediate aftermath of the shootings in Connecticut, the number of rural Americans who supported major gun restrictions rose to 49% but now that support has dropped 22 points," Holland said. "Support for stricter gun laws dropped 16 points among Americans over 50 years old in that same time."
However, Obama noted that polls also show strong support across the spectrum, including among gun owners, for expanding background checks to prevent firearms from ending up in the hands of the mentally ill and criminals.
The CBS poll showed that 90% of respondents agreed, and the president noted that support, asking Thursday "how often do 90% of Americans agree on anything?"
He urged people to find out where their congressional representatives stand on the gun legislation, adding that if a legislator isn't "part of the 90%, ask why not."