In the five years since 20 students and six educators were killed at Sandy Hook Elementary School, the Connecticut State Police have been unable to complete a report that law enforcement across the country look for when preparing for future incidents of extreme violence. These inward-looking investigations, known as after-action reports, examine the police response to individual mass shootings, acts of terror, or other major incidents of violence.

Multiple agencies have investigated and produced reports about the attack itself. And just more than a year after Adam Lanza killed his mother and then carried out the deadly massacre at the school, where he also killed himself, the Connecticut State Police released a 7,000 page investigation of Lanza's life and the shooting. But the status of its after-action report, which would likely include analysis of any mistakes made during the police response, is a mystery — as first reported by the Hartford Courant.

Former State Police Col. Danny Stebbins, who before his retirement in 2014 oversaw the investigation into the Dec. 14, 2012 shooting, said in a phone call with CBS News that a draft of the after-action report was completed at least two and half years ago.

"When you have a report of that size and production, they had been working on it for some time (when) I retired. I believe the report was done shortly after that," said Stebbins, who before his retirement was interviewed by troopers preparing the after-action report.

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Stebbins said the report has to be approved by Connecticut State Police commissioner Dora Schriro before it can be released.

"It's her shop. It's her agency," Stebbins said.

In response to questions from CBS News, a spokeswoman for the Connecticut State Police confirmed that a draft of the after-action report was completed, but that it has yet to be cleared by Schriro. The spokeswoman said the agency is unable to provide an estimated completion date, and wouldn't comment on which departments or troopers are still working on the report.

The delay is unusual, said John DeCarlo, a former police chief who is a professor of criminal justice at the University of New Haven.

"In any of the instances that I have ever heard of, when something like this happens, and there's a major incident like this, there is an after-action," DeCarlo said. "Now when you ask is five years a long time? Yeah, that's a real long time."

After-action reports are often submitted to the federal government's National Criminal Justice Reference Service, which allows police departments across the country to review and learn from the police responses to mass shootings.

DeCarlo said they're also intended for civilians.

"The public pays for the police, so there has to be some kind of accountability of how professional, how knowledgeable and how effective police are at their jobs," DeCarlo said.

Richard Fairburn, the Public Safety Director for the town of Canton, Illinois, remembers as many as 85 law enforcement agencies clamoring for an after-action report he helped produce for a state police agency that was involved in a 39-day standoff with a barricaded woman.

He said there's likely to be much more interest in the Sandy Hook report.

"Something of that significance, that's a huge nationally significant incident, there should be an after-action report," Fairburn said. "They can take quite a while. A year, probably would not be unreasonable. I certainly would hope it could be done sooner."

Fairburn noted that departments can sometimes avoid releasing the reports for fear of litigation, because they often acknowledge mistakes made.

"If that story's not out there, probably there's a reason it's not out there," Fairburn said, adding that he advises police departments to be straightforward in their after-action reports. "You can say, 'This is what did not go the way we would like, and this is what we would like to improve next time.'"

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