HARTFORD, Conn. — Investigators are planning to release a long-awaited report on the Newtown school shooting, nearly a year after the massacre of 20 children and six women inside Sandy Hook Elementary School.

The summary report by the lead investigator, State’s Attorney Stephen Sedensky III, could provide some of the first official answers to questions about the history of the gunman and the police response to one of the worst school shootings in American history.

The Dec. 14 shooting plunged the small New England community into mourning, elevated gun safety to the top of the agenda for President Barack Obama and led states across the country to re-evaluate laws on issues including school safety.

The report expected Monday afternoon will not include the full evidence file of Connecticut State Police, which is believed to total thousands of pages. The decision to continue withholding the bulk of the evidence is stirring new criticism of the secrecy surrounding the investigation.

Dan Klau, a Hartford attorney who specializes in First Amendment law, said the decision to release a summary report before the full evidence file is a reversal of standard practice and one of the most unusual elements of the investigation.