(CNN) — Connecticut authorities closed the book on last December’s massacre at Sandy Hook Elementary School on Monday without answering what drove the 20-year-old behind the rampage.
A 44-page summary of the second-deadliest shooting in U.S. history outlines how gunman Adam Lanza killed his mother, then stormed the elementary school and killed himself as police arrived. But investigators haven’t determined a motive for the assault, which left 20 first-graders and six adults at the grade school dead in less than 11 minutes.
“The evidence clearly shows that the shooter planned his actions, including the taking of his own life, but there is no clear indication why he did so, or why he targeted Sandy Hook Elementary School,” Monday’s report states.
Lanza “had significant mental health issues that affected his ability to live a normal life and to interact with others,” the report states. But it adds, “What contribution this made to the shootings, if any, is unknown as those mental health professionals who saw him did not see anything that would have predicted his future behavior.”
“With the issuance of this report, the investigation is closed,” it concludes.
Lanza had “an obsession” with mass murder and “a strong interest in firearms,” the report states. On his computer, investigators found a spreadsheet detailing mass murders over the years; two videos depicting gunshot suicides; two pictures of Lanza pointing guns at his own head; and movies and a video game depicting school shootings.
Another hard drive found in his home appeared to have been intentionally damaged, and investigators were unable to recover anything from the device. There were five guns in the home, four of which were found with Lanza and the fifth was used to kill his mother, Nancy Lanza.
The report is separate from a much longer evidence file that Connecticut State Police will release at an unspecified date. The family of Victoria Soto, a teacher who shielded her students before being shot to death, said the release is “yet another blow that our family has been dealt.”
A statement from the family said, “While others search for the answer as to why this happened, we search for the how. How can we live without Vicki? How do we celebrate Christmas without Vicki? How do we go on every day missing a piece of our family? Those are the questions we seek the answers for. There is nothing in the report that will answer those for us.”
Connecticut Gov. Dannel Malloy said the report’s release “will no doubt be difficult” for the relatives of those killed at Sandy Hook.
“But if there is one thing that I believe we must do, it’s that we must honor the lives that were lost by taking steps to protect ourselves from another horror like this,” Malloy said. “I hope that the information in this summary and in the supporting documents that will be released by the State Police takes us closer to that goal.”
Victims’ family members were informed of the report, said Mark Dupuis, a spokesman for Danbury State’s Attorney Stephen Sedensky, whose office conducted the investigation.
Dupuis declined to provide details about when, where, and how the families were given the details of the report.
“We are sensitive to the needs of the families, and those needs are being addressed,” Dupuis said.
CNN’s Susan Candiotte, Rande Iaboni and Ronni Berke contributed to this report.