The baby monitor didn’t go off. Any sound or motion in the twins’ room was supposed to set off an alarm — but in the wee hours of Nov. 27, 2022, Katie Czajkowski-Fell and Justin Fell weren’t woken up. They’d gotten it because of Hayden’s febrile seizures. These are common, generally nothing to worry about, the doctors said. Toddlers’ immune responses can be more assertive than adults’, burning into action against a world they’re still getting used to, routinely sparking high fevers, potentially irritating the brain. Hayden would likely grow out of them.

Still, Katie worried. Whenever he got sick, he began to convulse; she felt helpless, watching his eyes roll back. What if it happened during sleep? Hence the baby monitor, filming, storing footage, ready to trip an alarm.

They got up at a normal time, on a normal Sunday morning in Bel Air, Md., just outside of Baltimore. Justin went into his office, to change his corporate password, the way offices require you to every few months. Then he went to get the twins. Cameron was up, but Hayden’s crib was unusually still. Time stopped. He yelled, began CPR, called 911. The first cop who showed up flashed an expression of disbelief — “I think I see him breathing,” he said — but Hayden’s skin was going blue, his feet cold. Justin had just spent what felt like an eternity trying to breathe life back into him. “My child’s body felt like a piece of Styrofoam,” Justin said.

Besides the febrile seizures, Hayden had been a healthy 17-month-old, always ready to dance in that intuitive toddler way, shimmying, wiggling, whether his parents had put on Radiohead or Justin Timberlake. That kind of out-of-nowhere-ness is a hallmark of a syndrome known as sudden unexplained death in childhood: thriving one day, only to perish mysteriously during sleep, usually with no warning or witness. This time, though, there was a witness, albeit an electronic one: the baby monitor.

Hayden’s death is part of a pattern, described in a study published Thursday in the journal Neurology. In a registry of some 300 such cases, New York University researchers were able to find seven in which there was footage of the death, whether from baby monitors or “nanny cams” or security systems. In five of the recordings, including Hayden’s, experts saw motion that looked distinctly like some kind of convulsion. In the sixth, many thought they detected that same sort of movement, but weren’t all in agreement. The seventh was too grainy to tell.

The sample size is tiny, but the conclusions striking: video evidence supporting a long-suspected link between seizures and sudden death in young children. Hayden was the only one with a known history of seizures, yet something similar and seizure-like happened in the 30 minutes before most of these children died.

“The forensic pathologist’s job is to kind of work backwards. ‘What can I discover to understand what happened to that child?’” said Laura Gould, a research assistant professor at New York University and an author of the new paper. “It was the videos alone that implicated seizures. Otherwise, we would have not had evidence that seizures had occurred.” Often, that sort of electrical pattern leaves no mark, no clue to be found on autopsy. This series of tragedies is a case in point: None of their post-mortem exams had revealed a definitive cause of death.

Though the study leaves plenty of questions unanswered, to others in the field, this is an important step forward. “What’s difficult is, we were trying to discover a disease without a clinical manifestation,” said Rick Goldstein, director of Robert’s Program on Sudden Unexpected Death in Pediatrics, at Boston Children’s Hospital, who wasn’t involved in the study. “And here’s a clinical manifestation.”

In a way, this investigation began at a 1999 conference on sudden infant death syndrome in Atlanta, where Henry Krous, a renowned pediatric pathologist, was asked to present on what might be happening in cases that looked like SIDS, but didn’t technically count, because they’d taken place after age 1. It was a very short talk. “I actually didn’t know anything about the answer,” recalled Krous, now an emeritus professor at University of California, San Diego. In fact, neither he nor any other scientist knew much about these cases. He put the question to the audience. Did anyone know something? Were there any parents in attendance who’d lost a child?

“To my utter amazement, seven families raised their hands,” he said.

One of those people was Laura Gould. Her daughter Maria had been 15 months old when she went down for a nap and never woke up. It was 1997. The day before, she’d been toddling around their home in Montclair, N.J., learning new words. Gould thought that for sure, she’d missed something, some raging meningitis. She expected the autopsy report to be like the ones she’d seen on TV, the medical examiner calling with a definitive cause of death: case closed.

“I couldn’t believe it when they called me and said that in the initial autopsy, everything was normal,” she recalled. “‘We have no idea. We’re going to do more tests.’”

Though she was a physical therapist by training, she began working with Krous, collecting these painful stories, asking for medical records, taking detailed histories. They came up with the name for the syndrome, and an acronym: SUDC. Once they had 50 cases in their registry, they tried to put together a summary, an attempt at answering the question Krous hadn’t known anything about six years earlier. “I’ll never forget my research assistant running into my office,” Krous recalled. “She said, ‘Do you realize how often there’s a history of febrile seizures in the child that died, or the family, or both?’”

All told, they found that to be true 33% of the time. They knew these terrible mysteries were almost certainly heterogenous in cause: some undetected cardiac issues, some undiagnosed problems with the metabolic machinery of cells. Here was a hint that at least a significant portion of these children had neurological troubles. Researchers began asking grieving parents for donations of their deceased loved ones’ brains, to look for structural signs — and they found some, an unusual patterning of cells in the hippocampus, slight asymmetries possibly linked to epilepsy. Yet these remained correlations, associations, not always present even in cases where there was a history of seizures.

“It’s a little bit like you’ve been wondering who’s been robbing banks. And now you have a video of the culprit coming out of the bank with a bag of money,” said neurologist Orrin Devinsky, the director of New York University’s comprehensive epilepsy center, and an author on the new paper. “I wouldn’t say I know 100% that it’s seizures. But my goodness, it’s extraordinarily strong evidence.”

It’s possible, he went on, that these children had cardiac events, which cut off blood flow and thus oxygen to the brain, triggering convulsions. But arrhythmias typically happen during exercise, not during sleep; these children’s hearts showed no abnormalities; and sequencing the protein-encoding portions of their genomes didn’t unearth any relevant mutations.

Many questions remain. It’s hard to say what these videos can tell us about cases for which there aren’t any, besides that convulsions are a worthwhile hypothesis. It’s hard to say what exactly happened in these videos, and how convulsions and the state of slowed breathing that often follows them might be playing a role. It’s hard to say to what extent these episodes might be preventing some children from shifting position to clear something that might be blocking their breathing, the way the vast majority of kids do.

“The Holy Grail of all of this sort of thing would be: Could we predict ahead of time a baby who’s about to die? And if so, could we do something about it?” said Tom Keens, a pediatric pulmonologist at Children’s Hospital Los Angeles, who wasn’t involved in this research. “This does, theoretically, provide a pathway to that” — but it remains theoretical, far off.

It’s the blessing and the curse of incremental research: A useful hint for scientists, but providing little that is actionable for parents. They’re left with standard safe-sleeping guidelines and admonitions against smoking and suggestions of what to do if your child is having a febrile seizure: Seek immediate care the first time it happens, in case it’s meningitis or something else, and on subsequent occasions, lay the child gently on the floor with nothing in their mouth, and call 911 if it lasts more than a few minutes or happens repeatedly in the span of 24 hours.

High-tech devices contributed invaluable data for this new study, but doctors see little evidence that they help prevent SIDS or SUDC. It is the sad reality that infants occasionally die from SIDS even in the hyper-monitored environments of hospitals; epilepsy patients sometimes die suddenly in medical units specifically designed to analyze their illness, wired up with sensors tracking their every movement, breath, and heartbeat. Hayden’s convulsions were so subtle they didn’t set off the motion-triggered alarm.

Even the presence of video evidence did not save Hayden’s family from the tone of accusation that sometimes clouds these sorts of post-mortem investigations. A forensic photographer arrived. Child protective services was called. Their house felt like a crime scene.

They whisked their older daughter to her grandfather’s, who made her waffles, tried to shield her from the awfulness that had suddenly torn into their home. Justin pulled up the video, figuring it might hold some clue as to what happened — and sure enough, when he slowed it down, he saw Hayden’s movements change, saw his son take his last breath around 2:06. Yet somehow an officer mistakenly wrote in a police report that Justin had woken up, watched his son convulse, and then went back to sleep — a suggestion he and Katie found sickening. If only the alarm had woken them up. If only they had been there at his bedside to call 911 the way they had for previous seizures.

They weren’t allowed to carry Hayden out of the house. There is no description for the mind-bending depths of grief that come with losing a child. There’s a numbness to everything, as if you’ve exited reality. One day Hayden had been there, intent on playing when he was supposed to be sleeping, keeping his twin Cameron awake, sometimes grabbing the baby monitor so that the footage wobbled across the wall and the curtains. “Like a dog with a GoPro,” Justin said. They’d go scoop him up, snuggle him a little bit longer until he was ready to sleep. Then he was gone. Katie couldn’t dress herself the day of his funeral.

The research wasn’t just a way of finding some glimmer of an answer for the inexplicable, but also hopefully a way of moving toward a moment in which no parents experience this kind of loss. As Laura Gould put it, “People say, you know, ‘You do this for Maria. And I say, unfortunately, I can’t do anything for Maria right now. But we can do a whole lot for the children who are still living.’”

Families who sign up their children for her registry have the same impulse. The conversations involved are painful, tearful — but also, in some ways, therapeutic, to talk to someone who knows what it’s like, to be able to talk about the kid they miss so deeply. “I used to turn off the lights and go ‘Okay, guys, good night.’ And then I would turn it back on real quick and go, ‘I love you!’ And they would laugh and laugh and laugh. And I would do it a couple times,” Katie said.

Cameron doesn’t sleep there anymore. His crib is near his parents’ bed. But Katie still goes into that empty room, turns the light on, says “I love you.” She can’t not do it. “I say goodnight to my other kids. I feel like I need to say goodnight to him, too.”