(CNN) -- The first frantic call comes from the school secretary inside the main building.
"Sandy Hook school," she says. "I think there's somebody shooting in here. In Sandy Hook school."
"OK, what makes you think that?" the dispatcher asks.
"Because somebody has a gun. I saw a glimpse of somebody running down the hallway."
"They're still running, they're still shooting," the caller exclaims. "Sandy Hook school please!"
Audio recordings of 911 calls released Wednesday -- nearly one year after the second-deadliest shooting in U.S. history -- offered a glimpse into the horror that day last December at Sandy Hook Elementary School. The recording also revealed the composure of the 911 dispatchers who received the first calls for help.
Town dispatchers are heard calmly responding to tense calls from school staff. Dispatchers assure them help is on the way and ask about the children. Staff members are told to stay inside the school and to take cover. Gunfire is heard in at least two of the calls.
The December 14, 2012, massacre left 20 first-graders and six school employees dead, and galvanized calls for stricter gun control throughout the nation.
The first 911 call was made about 9:35 a.m. -- with the first police officers arriving at the school less than four minutes later, investigators said.
The shooter, Adam Lanza, 20, shot himself less than five minutes after that call. An investigative report released last month described Lanza, a former Sandy Hook student, as an antisocial young man obsessed with mass killings.
Police said Lanza shot his mother, Nancy, sometime between 8 a.m. and 9 a.m., using a .22-caliber rifle.
Then Lanza got into his car and drove a few miles to the school -- armed with a Bushmaster rifle, a Glock 10-millimeter pistol, a Sig Sauer 9-millimeter pistol and a large supply of ammunition, investigators said. Lanza shot open the glass window on the lobby door at the school.
The second call comes from school custodian Rick Thorne.
"I believe there's shooting at the front glass," Thorne says. "There something going on."
"Okay I want you to stay on the line with me," the dispatcher counsels. "Where are you in the school?
"I am down the corridor."
"All right I want you to take cover ... All right, get everybody you can going down there. "
"The front glass is all shot out..."
"... What about the students in the building?"
"Everything is locked up as far as I know."
The dispatcher keeps his composure as the staff member reports that the shooting is continuing. "No it's still going on," the custodian reports. "I can't get over there."
"I don't want you to go over there. I want to know what's happening with the students, though ... Do you see anything or hear anything more?"
"I keep hearing shooting," the custodian says. "I keep hearing popping!"
The release of the 911 details Wednesday involved calls made on landlines from the school to police in Newtown. Other calls, including those routed to other law enforcement agencies, were not part of the release.
Last week, Connecticut Superior Court Judge Eliot Prescott upheld the Connecticut Freedom of Information Commission's ruling to release calls related to the shooting.
A state attorney had tried to block the release to shield the victims' families; the Associated Press had challenged authorities' refusal to release the 911 tapes.
Wednesday's release, which was supervised by town attorneys, came in the wake of local opposition, and the judge's ruling. And the release left news organizations with hard choices difficult about what to broadcast from the events of a shattering day in the lives of a small, quiet town and an entire nation.
Neil Heslin, whose 6-year-old son Jesse Lewis was killed in the rampage, has listened to parts of the 911 tapes.
"I wish they weren't released. It's just a sad reminder of what happened, and clearly the day is etched in my mind," he told CNN's Piers Morgan.
The shooter's aunt, Marsha Lanza, questioned the timing of the release so close to the one-year anniversary.
"I think emotions are very raw and (it's) too soon," she said. "I really believe they needed to wait."
Investigators have said that Lanza entered the school and moved down the hallway. His first victims were Principal Dawn Hochsprung and school psychologist Mary Sherlach, who had heard glass shattering and "loud banging," according to the report. They went into the hallway, with Hochsprung warning a third staff member to "Stay put!" Both were shot to death. The staff member was also shot but survived and called 911.
At one point, a female dispatcher tells her colleagues of the mass shooting: "Guys, we got a shooting at Sandy Hook elementary school in Newtown! That's why 911 is ringing off the hook."
Another dispatcher handles a call from a man inside the school.
"There's still shooting going on. Please!" the caller exclaims. A banging sound is heard.
"What about injuries at this time?"
"I don't know of any injuries right now."
"OK," a dispatcher is heard telling a colleague, "I need you to call state police..."
A gunshot blast is heard.
"It's still, still going on!" the male caller says.
A teacher calls from her classroom.
"It sounds like there are gunshots in the hallway," she says.
"OK, do you have everyone in the classroom and the door is locked?" the dispatcher asks calmly.
"All of my students, the door is not locked yet. I have to go...I have to go lock the door ... "
"Keep everybody calm, keep everybody down. Get everybody away from the windows, OK?"
Some staff members took shelter in the school's main office as Lanza moved toward two classrooms where he killed 20 children and four adults. Others protected and tried to calm their students.
"In fewer than 11 minutes, 20 first-grade pupils and six adults had lost their lives," the report said.
The killings in Newtown, about 60 miles outside New York, happened less than five months after a similar bloodbath at a movie theater in Aurora, Colorado, outside Denver.
Those mass slayings triggered a nationwide debate over gun violence, school safety and mental health, a debate that produced some new restrictions on firearms in several states. A backlash by gun-rights advocates followed
A new CNN/ORC International survey found that 49% of Americans say they support stricter gun control laws, with 50% opposed. The 49% support is down six percentage points from the 55% who said they backed stricter gun control in CNN polling from January, just a few weeks after the tragedy at Sandy Hook Elementary School
CNN's Chelsea J. Carter contributed to this report.