To try to figure out the motive, investigators said, they interviewed members of Lanza's family — his father and brother cooperated fully — along with teachers and others. They said they also tried within the limits of privacy laws to gather information on his medical treatment.
They found no evidence the young man had taken any medication that would have affected his behavior or explain the bloodbath.
Sednesky's report is a summary of a much larger Connecticut state police evidence file that is expected to be released at a later date.
Sedensky has gone to court to fight release of the 911 tapes from the school and resisted calls from Connecticut's governor to divulge more information sooner.
The withholding of 911 recordings, which are routinely released in other cases, has been the subject of a legal battle between The Associated Press and Sedensky before the state's Freedom of Information Commission, which ruled in favor of the AP, and now Connecticut's court system.
A Connecticut judge said Monday he will listen to the 911 recordings from the school before ruling on whether they can be publicly released.
If the recordings are released, the AP would review the content and determine what, if any, of it would meet the news cooperative's standards for publication.
The report said the first officer arrived behind the school at 9:39 a.m. Two other Newtown officers then arrived at the school within seconds, and gunshots were heard in the background.
The last gunshot officers heard, which is believed to be the suicide shot by Lanza, was heard at three seconds past 9:40. Newtown officers entered the school at 47 seconds past 9:44, according to the report.