The Danbury State’s Attorney closed the investigation into the Newtown shooting Monday, concluding that the shooter acted alone in his deadly assault on the elementary school. But in the initial chaos, police worked as if another shooter was on the premises.

Danbury States Attorney Stephen Sedensky III released his final report Monday on the Dec. 14, 2012, shooting at Sandy Hook Elementary School (SHES). Avoiding the use of the names of many of the witnesses or victims involved, Sedensky’s report helps to shed light on what transpired during and after the incident that left 20 school children, six educators, and the shooter’s mother dead, as well as others wounded at the school.

The report details a timeline of 20-year-old shooter Adam Lanza’s attack on the school and of communications between first responders. According to the report, the first police officer arrived at the school within four minutes of the first 911 call, and officers entered the school about six minutes later. Lanza shot and killed himself within one minute of the first police officer arriving at the scene, about 4 to 5 minutes before officers entered the school.

In the report, Sedensky concludes that Lanza acted alone and says none of the evidence gathered points to the existence of any co-conspirators. He points out that it’s not clear which classroom Lanza entered first, but what the report appears to show is that regardless of whether officers had plunged through the doors within seconds of their arrival, most if not all of the damage had already been done.

However, some early encounters between officers and bystanders during the incident led police to believe they could have been dealing with more than one shooter.

According to the report, the first Newtown officer arrived at the scene at 9:39 a.m. — that’s about 9 minutes after the shooter began his rampage but only about four minutes after the first 911 call. Within a minute of arriving, a Newtown police officer encountered a man running alongside the building carrying something in his hand.

“From the time the unknown male was encountered by the Newtown police outside of SHES until after the staff and children were evacuated, all responding law enforcement operated under the belief that there may have been more than one shooter and acted accordingly,” Sedensky wrote.

The first man turned out to be a parent carrying a cellphone, but until police could determine his identity and reason for being on the scene, he was handcuffed and treated as a suspect.

According to the state police timeline of communications, the officer used his radio to inform a supervisor that he had detained a man, which led to an advisory that a secondary shooter may be involved. The transmission also drew the attention of other Newtown officers who had responded to the scene.

Police detained at least three other people outside the school, according to Sedensky’s report.

“Two reporters located in the woods around SHES, who were held at gunpoint by Department of Energy and Environmental Protection (DEEP) police officers until their identities could be determined,” he wrote.

The other man detained by police was a Samaritan from New York who was in the area and wandered onto the scene on foot after an automated emergency response message on his cellphone alerted him to a situation at the school.

Peter Massey, a former police detective and head of the undergraduate program in forensics at the University of New Haven, said the possibility of a second shooter is always on the minds of first responders to such shootings. He said tracking down those leads takes time.

“They’re making big judgments in seconds or milliseconds,” he said. “I think they did the best job they could with the information they had at the time.” 
Even after Lanza had shot and killed himself, police were positioned as lookouts in the areas surrounding the school while students and staff were being evacuated from the building. Police dogs also were brought in to search the area.

According Sedensky’s report, the notion that there may have been several shooters continued to be a consideration even after Dec. 14. 
“It was only after potential leads were investigated that investigators became confident that the shooter was not aided in any way by others and that no one knew of the shooter’s plan prior to Dec. 14, 2012,” he wrote.

Lanza was described by many as a loner who liked to play video games. He seemed to have an obsession with school shootings and firearms.

When state police searched Lanza’s home they found a copy of “Amish Grace.” The book details the 2006 school shooting in Pennsylvania. There also were old newspaper articles detailing a school shooting in 1891. Both were in a drawer in the computer room where Lanza had smashed his hard drive and where the gun safe was located.

The gun safe contained an “Enfield” Albion bolt action .303 caliber rifle and was open when police arrived. There also were numerous boxes of ammunition strewn around the closet and the safe. There was black tape on the windows.

Lanza’s computer included images of mass murders, which were broken down into categories by number of victims killed. It also contained several video clips of Eric Harris and Dylan Klebold, the pair who carried out the Columbine High School shooting. Police also found a Word Document of a screenplay that was presumably written by the shooter about four male characters who have a delusional discussion and all but one of them ends up being killed.

But none of the evidence collected at either crime scene seemed to point to a motive.

“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, despite the collection of extensive background information on the shooter through a multitude of interviews and other sources,” Sedensky said in his 43-page report.

“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.”

Lanza’s toxicology report did not show any types of drugs in his system at the time of his death.

“It is known that the shooter had significant mental health issues that affected his ability to live a normal life and to interact with others, even those to whom he should have been close,” Sedensky said. “As an adult he did not recognize or help himself deal with those issues. 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.”

Earlier Monday, a New Britain Superior Court judge agreed to listen to the 911 tapes from the day of the shooting in order to determine whether they should be released publicly under the state’s Freedom of Information Act. His decision is expected later this week.

Gov. Dannel P. Malloy has been critical of state prosecutors for failing to release the report sooner, but was confident it would provide enough information so that an advisory commission he created to make recommendations in response to the murders would be able to complete its work.

Christine Stuart contributed to this story.