"There are a lot of causes of death that don't have specific signs," said Kimberlee Moran of Rutgers.
LINDENWOLD -- Dr. Kathryn McCans has become familiar with the myriad, tragic ways that infants can die.
She is an emergency room pediatrician with a focus on child abuse cases, and for the last two years she's chaired the state's Child Fatality and Near Fatality Review Board.
She said some causes leave obvious evidence, like bruises or positive toxicology results, while others are harder to determine. Sometimes, they remain a mystery.
Officials are currently combing over evidence and compiling autopsy results to determine whether there are any signs as to what killed 7-month-old twin girls. They were found not breathing by their mother in an apartment in Lindenwold Wednesday morning.
McCans said she is not familiar with the investigation into the deaths, but she is sure of one thing: "The fact that they died on the same night is very unusual."
And while some may jump to the conclusion that that points to foul play, McCans said there are many reasons two infants asleep in a crib could die. It is unlikely but not impossible that natural causes could have killed them nearly simultaneously, she said.
"It is possible that this is a very rare occurrence," she said.
The Camden County prosecutor's office said that an autopsy was performed and investigators will not be getting an official cause of death until various test results come back from the lab.
NJ Advance Media talked to McCans and Kimberlee Moran, director of forensics and associate teaching professor at Rutgers University - Camden, about what investigators will be looking for as they try to determine how the twins died.
What we know
The twins' mother, identified by 6abc as Alexandria Garnett, frantically told a 911 dispatcher that her children were not breathing and were "turning purple."
Turning purple or blueish is a sign of oxygen deprivation, Moran said.
McCans said that a purple skin color could also be due to lividity -- blood pooling in whatever part of the body is facing downward -- if the children had been dead for a while. The mother told the 911 dispatcher that the babies were on their stomachs when she found them.
The coloring also helps rule out carbon monoxide poisoning. People who die from inhaling too much carbon monoxide have too much oxygen in their blood, which makes their lips turn a bright, cherry red, Moran said.
"The fact that there's a blue color and not a cherry red probably means it wasn't carbon monoxide," Moran said.
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Lindenwold Fire Chief Michael Nolan also said Thursday that if there had been any chance of carbon monoxide poisoning, his firefighters would have been called to use meters to test the levels in the apartment. They were not.
To determine what might have caused the infants to stop breathing, investigators would likely rely on autopsy results as well as evidence collected at the scene, Moran said.
The medical examiner would look for bruises and tiny, burst blood vessels in the eyes as possible signs of strangulation or constriction of some kind, Moran said.
Neither would likely be present if a child died from having her airway blocked, either purposely or inadvertently, she said.
Infants that are too young to lift their heads -- which they generally can by 7 months -- could suffocate if their faces end up against blankets, stuffed animals or other items in a crib, McCans said. Still, she said, the chance of both babies suffocating is unlikely.
Environmental factors
McCans said that in cases where children die together in the same room, investigators are probably going to check for environmental hazards, including toxins.
That could mean carbon monoxide poisoning or tobacco or drugs, if someone was smoking something near the infants.
"Because they're the smallest children, they would be affected at lower levels," she said.
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While babies can rollover and move a little by 7 months old, she said the chance that the babies could get into something toxic by themselves isn't good. However, there are instances where children have inadvertently consumed drugs, alcohol or other things that are toxic to them.
Toxicology tests could show if a child has consumed a drug or toxin, whether it was intended or not. McCan said there are cases where parents give their children cold medicine or other medication not meant for children because they want them to sleep, and it can kill them.
What else is possible?
McCans said it is also possible but unlikely that an illness, infection or virus caused their deaths. "It would be pretty unusual for them both to get sick and die in the same night," she said.
Twins are more likely to be allergic to the same things, McCans said, but anaphylaxis, the most deadly kind of allergy, doesn't usually occur until a child is older.
Asked whether they could have had reactions to vaccinations, McCans said that the fear of babies having serious reactions to vaccines is unfounded. Plus, most babies are vaccinated at 6 months, not seven.
"Fatalities from vaccines are incredibly, incredibly rare," she said. "I would not expect that vaccines were involved in these deaths."
Authorities have not said whether they suspect that child abuse caused the deaths.
Moran said a medical examiner performing an autopsy on a dead child will definitely be looking for signs of abuse. The autopsy may reveal bruises, broken bones or physical damage to organs. In a child that has been shaken, the brain will sometimes swell, she said.
Other evidence, including interviews and things collected from the scene, could help rule out or firm up any theory investigators are working on, she said.
"It's so important for the investigation to have good documentation of the scene, very thorough and methodical," Moran said.
And unfortunately, forensics is not a perfect science, she said. The autopsy and the whole investigation could be inconclusive.
"There are a lot of causes of death that don't have specific signs," Moran said. "A lot of the determination is based on exclusion -- what isn't there."
Rebecca Everett may be reached at reverett@njadvancemedia.com. Follow her on Twitter @rebeccajeverett. Find NJ.com on Facebook.