(CNN) -- Police released new documents related to the shootings last year at Sandy Hook Elementary School in Connecticut, but a motive for the attack by the troubled young man remained elusive.
The information released Thursday shows a gun safe was found in the bedroom of 20-year-old Adam Lanza, who shot his mother, Nancy, in her forehead as she lay in bed.
He then took weapons belonging to her and drove to the school in Newtown, where he gunned down 20 children and six staff members before killing himself.
Investigators found more than 1,600 rounds of ammunition in the house, and a holiday card with a check "made out to Adam Lanza for the purchase of a C183 (firearm), authored by Nancy Lanza," according to a search warrant.
The warrant cited an interview with a person whose name was redacted who said Lanza rarely left his home and that he was a shut-in "and an avid gamer who played Call of Duty, amongst other games."
Some of the documents, released by state prosecutors, have been redacted at the prosecutors' request, said Paul Vance, a spokesman for the Connecticut State Police. Relatives of the victims were briefed Wednesday about the documents.
State police are continuing their investigation, which is not expected to be completed until June, he said. "The information revealed today underscores the need to turn this tragedy into transformation," said Tim Makris, co-founder of Sandy Hook Promise, which seeks to prevent a recurrence of such shootings.
"While legislation is not the only answer, it's time for Congress to pass sensible measures supported by the vast majority of Americans to reduce gun violence."
Also Thursday, Mayors Against Illegal Guns released the first television ad featuring relatives of the Newtown shootings. In the spots, relatives of the victims call on political leaders to pass tougher gun laws.
"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," said New York Mayor Michael Bloomberg, co-founder of the group, in a news release.
Recent polling, however, shows public support for gun restrictions has declined since the shootings, as renewed attention has amplified on both sides of the debate.
Shortly after the school massacre, 52% of Americans favored major restrictions on guns or making all guns illegal, according to a CNN/ORC International poll. That number has dwindled to 43%, according to the same poll, which was conducted March 15-17.
The killings have led Connecticut legislators to re-examine the state's gun laws, which are among the nation's strictest, and have reopened a national debate on gun control.
U.S. Sen. Richard Blumenthal, D-Connecticut, has announced plans to introduce legislation requiring background checks to purchase ammunition. He has pressured members of his state legislature to take action to bolster his efforts on a national level.
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