No motive found in Sandy Hook massacre WHY a young gunman went on a murderous shooting rampage at a US school a year ago is still a mystery and may never be known with certainty, prosecutors say in a report that ended their investigation. Adam Lanza, 20, was obsessed with mass murders and the 1999 Columbine High School shooting in particular, but investigators did not find evidence he ever told others of his intentions to carry out such an attack, according to the summary report released on Monday by the lead investigator, State's Attorney Stephen Sedensky III. Lanza killed 20 young children and six educators with a semi-automatic rifle inside the Connecticut school on December 14. He shot and killed his mother inside their home before driving to the school, and killed himself with a handgun as police arrived. The shooting moved gun safety to the top of the agenda for President Barack Obama - though his gun control efforts have failed in congress - and led states across the country to re-evaluate laws on issues including school safety. The obvious question that remains is: 'Why did the shooter murder twenty-seven people, including twenty children?' Unfortunately, that question may never be answered conclusively," the report said. Sedensky also said there was no clear indication why Lanza chose the school as the target other than the fact that it was close to his home. In a footnote, Sedensky said a computer drive recovered from Lanza's home might include potentially important evidence, but is unreadable, and it is highly unlikely any data will ever be extracted from it. The report said Lanza had "significant mental health issues" - in 2005, he was diagnosed with Asperger's disorder - but "what contribution this made to the shootings, if any, is unknown." Asperger's is an autism-like disorder that is not associated with violence. Lanza "was undoubtedly afflicted with mental health problems; yet despite a fascination with mass shootings and firearms, he displayed no aggressive or threatening tendencies," Sedensky wrote. Some recalled that the shooter had been bullied; but others - including many teachers - saw nothing of the sort. The report also says nearly six minutes passed between the arrival of the first police officer and the time officers entered the school. A timeline released with the report says the first officer arrived behind the school at 9.39am. The last gunshot officers heard, which is believed to be the suicide shot by Lanza, was heard at three seconds past 9.40. Officers entered the school at 47 seconds past 9.44.