SURFSIDE — The thunderous bang that jolted Jonah Handler and his mother in the middle of the night last June was followed by silence.

Jonah, 15, and his mother, Stacie Fang, went out to their terrace and looked up, thinking the ominous sound had come from the roof of the 13-story Champlain Towers South condominium in Surfside, Fla. But standing on the 10th floor, they could not see anything wrong, so they settled back down for the night.

All was quiet. No alarm blared. No evacuation order came. But the condo tower was on the precipice of collapse.

One year since the catastrophe at Champlain Towers, with the cause of the collapse still under federal investigation, new documents, interviews and deposition records have shed fresh light on a critical seven-minute period between the roaring initial failure of a pool deck and the eventual cascading collapse of a portion of the building, leaving 98 people dead in one of the deadliest structural failures in U.S. history. The security guard in the lobby of Champlain Towers hurriedly dialed 911 to report the initial failure. An alarm may have sounded at that point in a limited part of the building, though it was clearly inaudible to many of those who still slept. The building also had a sophisticated audio warning system designed to broadcast an alert into the bedrooms of every unit. But it was never triggered, newly available deposition testimony and interviews show, because the security guard had never been trained about the system and the single button needed to activate it. “If I had known about it, I would have pressed it,” the security guard, Shamoka Furman, said in an interview.

The collapse happened around 1:22 a.m., when most residents were asleep.

The performance of the building’s automated fire alarm system remains one of the many frustrating questions still unanswered 12 months after the collapse. With seven minutes elapsing between the time of the pool deck failure and the catastrophic fall, could some of the residents who slept through the initial boom have been able to make their way to safety?

In the portion of the building that eventually collapsed, nearly everyone was killed, including Jonah’s mother in Unit 1002. Jonah, pulled from the rubble, miraculously survived with 12 broken vertebrae.

He said he never heard an alarm of any kind, and no alarm can be heard ahead of the collapse on any of the audio and video recordings that have emerged in the wake of the disaster.

Jonah’s father, Neil Handler, who was not in the building, said he was convinced that with seven minutes of warning, Jonah, his mother and any number of others would have been able to escape.

“I just think of all the lives they could have saved,” he said.

On Thursday, Judge Michael A. Hanzman of the Circuit Court in Miami-Dade County gave approval from the bench to a more than $1 billion settlement involving insurance companies, developers and other parties tied to Champlain Towers. Securitas, a company with a global footprint that was hired to help ensure the security of the building, paid the largest portion of the settlement — more than $500 million.

Before the emotional hearing began, the judge held a moment of silence to honor the victims. Relatives and survivors, sitting quietly in the courtroom, passed around tissues.

Securitas said in a statement that its participation in the settlement did “not reflect responsibility for the collapse of the building or the tragic loss of life.”

‘You press one button’

High rises have various ways to notify tenants of an emergency. Some older structures may have a basic fire alarm system that blares through the units. Many towers built in recent decades have added speakers so residents can get an audible command and a description of the crisis.

In the lobby of Champlain Towers, with its gleaming floors, recessed lighting and potted plants, a security desk contained the controls to a network of speakers that had been installed in every bedroom in 2017 to ensure that residents could be roused if an evacuation were needed. “All call” commands could be issued through a microphone at the lobby control panel.

“You press one button, it would power up every speaker throughout the building,” Matthew Haiman, who led the company that installed the system at Champlain Towers, said in a deposition. “You grab the microphone, you say: ‘Hey, guys, there’s an emergency, get the heck out of the building.’”

Had the system been used properly, he added, “then it probably would have saved more lives to be honest with you.”

Ms. Furman, who had been a Champlain Towers security guard for four months, said in an interview that she received minimal training when she was hired, with another security guard explaining the contours of the job while they stood for an hour in the lobby. She said she never learned about the “all call” button. The other guard declined to comment.

Andre Vautrin, a manager with the security company Securitas, said in a deposition that his company never trained the security guards at Champlain Towers how to operate the panel and implied that the condo management association oversaw the building’s security protocols.

A lawyer for families of the victims, Judd G. Rosen, queried further: “Do you agree with me that a reasonable security company should train its officers how to use a system that can notify all the residents of an impending disaster?”

“Yes,” Mr. Vautrin replied.

‘We decided to run’

The loud noise that brought Jonah and Ms. Fang to their balcony in the initial stages of the disaster also awakened Paolo Longobardi on the third floor. Thunder, he thought. But his wife, Anastasiya, had heard something more unsettling: an unnatural, metallic crunch.

The two of them, groggy with sleep, peered out the sliding glass door of their bedroom overlooking the pool. Below them, the pool deck was caving in.

“It was disappearing into the ground,” Mr. Longobardi said. “It was like a wave coming from the right to the left — from the south to the north — and it was falling.”

Around that time, the building’s alarm system was starting to activate, first at 1:15:29 a.m., when it signaled “trouble,” according to a data log. Seventeen seconds later, a fire alarm triggered. It sent out an automated alert to a monitoring company, though it is not clear that it generated an audible alarm on any floor. Soon after, a staff member at the monitoring company notified 911 that a fire alarm had been activated at Champlain Towers.

But even as initial signals of trouble were transmitted to the monitoring agency and then to the authorities, few people in the building were notified of what was happening.

The building’s default alarm system was not designed to alert every resident. Rather, an alarm that triggered on one floor was also supposed to set off alarms only on the floor above and the floor below. It remains unclear which alarms in the building went off that morning, and most survivors reported hearing no alarm. This included some of those who lived near the ground level of the building, where the initial failure occurred.

As he watched the pool deck collapse from Unit 309, Mr. Longobardi, a civil engineer who builds bridges for a living, thought a huge sinkhole might be swallowing the parking garage beneath the deck.

“We decided to run,” he said.

The Longobardis woke up their two children, ages 14 and 9, and ushered them out the door. Mr. Longobardi said one of the children recalled hearing an alarm during the escape.

In Unit 111 on the first floor, the Nir family, who had not yet gone to bed, also saw trouble at the pool deck and ran for the lobby. Gabriel Nir said he did not recall hearing a fire alarm, but his family urged Ms. Furman, the security guard, to call 911.

Ms. Furman dialed. The first call came in at 1:16:27 a.m., 41 seconds after the fire alarm had been triggered.

“A big explosion,” she reported. No alarm could be heard in the background of the call.

‘There was silence’

Six stories up, in Unit 611, Iliana Monteagudo awoke from her sleep, worried that she might not have closed her balcony door. Sure enough, it was open.

But as she went to close it, she found that the door was stuck. No alarms sounded in her room, but she could hear the sound of car alarms in the distance. Then she heard a crunching sound and saw a crack growing down from her ceiling.

“Run,” a voice in her head told her.

Ms. Monteagudo, 64, slipped out of her nightgown and into a dress — “Don’t waste time putting on a bra,” the voice told her — and sandals. She blew out a candle of the Virgin of Guadalupe, grabbed her keys, purse, credit cards and pill box, and ran out the door, being careful to turn off the lights behind her.

Out in the hallway, Ms. Monteagudo, who had moved into the complex six months earlier, was struck by how quiet things were. She assumed that the units around her were largely vacant. There was no alarm.

“There was silence,” she said. “There was no movement. Nothing. I thought the building was empty.”

With silence on their floor and no sign of a building in distress, Jonah and his mother returned inside their unit. He climbed back into bed to go back to sleep. She sat on the edge of his bed.

A little after 1:22 a.m., nearly seven minutes after the fire alarm system had triggered, the collapse turned 13 stories into a heap of rubble.

Mr. Nir was on a call with 911 and ran to safety. Ms. Monteagudo managed to reach a stairwell before the building fell around her, climbing out with the help of the security guard.

But Jonah and his mother never left his bedroom.

The floors of Champlain Towers pancaked on top of one another, leaving just inches between some of them; a rescuer later told Mr. Handler that the concrete on top of Jonah formed an A-shaped frame over his head, which is what probably allowed him to survive. A man walking by saw Jonah’s arm poking out of the rubble and fingers wiggling. He and another bystander alerted emergency workers.

The rescuer told Mr. Handler, who provided the account of Jonah’s survival for this article, that Jonah and Ms. Fang, who was 54, had been found holding hands.

“When he was separating them, they didn’t want to let go of each other,” Mr. Handler said.

Mr. Handler said that after that day, Jonah suffered from paralyzing fear when he heard sounds that reminded him of the collapse — especially thunderstorms. Mr. Handler sometimes has to drive his son around for hours until it stops raining.