Local officials in Newtown, Connecticut have cautioned residents to prepare themselves emotionally for Wednesday's release of nearly half an hour of 911 recordings from the shooting that killed 20 children and six adults at Sandy Hook elementary school last December.

Town officials initially tried to prevent release of the recordings of emergency phone calls made after Newtown resident Adam Lanza, 20, shot his way into the school. Lanza unleashed one of the worst mass shootings in US history before turning a gun on himself.

The state Freedom of Information Commission ordered the release of seven calls placed from inside the school. Gunfire is audible in the background of the audio-enhanced recordings, according to a lawyer for Newtown.

Late last month, a judge ruled the town must comply with the commission's order, and Newtown officials have since dropped their appeal. First Selectman Pat Llodra recently reversed her long-standing position, saying the tapes should be released in full in order to prevent partial leaks of the recordings.

On Tuesday, Newtown school superintendent John Reed emailed parents to alert them to the recordings' release and remind them the recordings could serve as an "emotional trigger".

Last week, Connecticut's state attorney, Stephen Sedensky, released a report on the Newtown massacre that concluded Lanza had acted alone, but that his motivation for the attack "may never be answered".