By Susan Candiotti and Ben Brumfield
(CNN) -- Police released new documents related to the shootings last year at Sandy Hook Elementary School in Connecticut, but a motive for the attack remained elusive.
Included in the new information is the report that a gun safe was found in the bedroom of 20-year-old Adam Lanza, who took guns belonging to his mother, Nancy, and shot her in her forehead in her bed. Then he went to the school in Newtown, where he gunned down 20 children and six staff members before killing himself.
Investigators found more than 1,400 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.
Relatives of the victims were briefed Wednesday about the documents, which were released Thursday morning.
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.
State police are continuing their investigation, which is not expected to be completed until June, he said.
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.
CNN's Susan Candiotti and Samira Jafari contributed to this report.