Romer and Jayson took her home on April 11 after flying to Arizona where they met baby Diana in hospital for the first time
Diana Sofia De Los Santos was found unresponsive in the SUV outside her parents' home in Santee, San Diego, about 12.20am on June 13.
She had been left in the car for several hours in 63F weather when a family member found her and called 911. The baby girl was rushed to hospital but did not survive.
San Diego County Sheriff's Office detectives are investigating how Diana was forgotten in the car and who left her there. No charges have been filed.
Studies show temperature inside a car can rise to 104F in just half an hour on a 70F day, and reach 115F in an hour. The body's organs begin to shut down at 107F.
Diana's parents Romer and Jayson De Los Santos took her home on April 11 after flying to Arizona where they met the baby girl in hospital for the first time.
Adorable photos showed them cradling the infant and introducing her to their two-year-old son, who was also adopted, and Diana at home in her crib.
'I haven't slept in days,' Romer wrote next to one of them.
The couple posted a link to an adoption site profile on social media in November, seeking a second child.
'Our hearts and home are open to providing more love to another amazing child,' they wrote on the profile.
'We want our children to grow up full of memories of playing on the beach, building sandcastles, making friends, biking to the park, going on Hawaiian cruises and learning how to raise foster kittens.'
Romer, a senior consultant at Jama Software, and Jayson, a stay-at-home dad since they adopted their son, have been together for more than 20 years.
They tied the knot in 2008 during the few months when same-sex marriage was legal in California before they were again banned by Proposition 8 until 2013.
The adoption profile described their lives in detail and promised to 'raise healthy, responsible, and most importantly, kind adults'.
Comment: Meanwhile this incident reflects neglect in the extreme.
The couple live in a three-bedroom house that has so many animals it is 'basically a zoo' with three cats, two dogs, chickens, fish, and a flock of parakeets.
They include a Samoyed named Hero that their son often sleeps on, a west highland white terrier, cats named Shego, Selina, and Puffy, and various foster animals.
Romer is an avid gardener and grows fruit, vegetables, bonsai trees, and flowers in the backyard and Jayson looks after the animals and collects eggs from the chickens.
'Our house is very popular with our neighbors and their children. They say it's like a petting zoo... Everybody is so excited to meet our future child,' they wrote.
The couple wrote that the new baby would have three 'ready-made best friends' in three boys aged less than two who lived next door and often played with their son.
'Seeing how gentle he plays with them we know that he would treasure a little sister or brother, and they will thrive with such a loving big brother,' they wrote.
The profile explained they both came from big families that often had dinners and parties and went on holiday together.
Romer and Jayson promised the birth parents could visit the child and see as many photos as they wanted.
'We admire the courage it takes for you to search for just the right family for the new life you are carrying. We recognize that it is not a decision to be taken lightly and understand that love is the source of your selfless act,' they wrote.
'We believe we're not only adopting a child, but also opening our hearts to a whole new family.'
A neighbor told DailyMail.com the couple seemed like good parents and were often seen playing outside with their children.
'They seem like a nice family who wanted to give a couple of kids a good life, it's just a shame it didn't turn out that way,' they said.
Paramedics rushed to the scene after 911 was called and tried to save Diana, who was rushed to Sharp Grossmont Hospital, where she died.
Neighbors said the ambulance came and left within a few minutes, and doorbell camera footage showed the late-night emergency response.
The county Medical Examiner's Office will determine the cause and manner of death.
About 37 children die from being left in a hot car every year across the US, including at least 56 in California since 1998, according to statistics by San Jose State University Adjunct Professor of Meteorology Jan Null for No Heat Stroke.
Comment: It seems that it's often those who have serious addictions, psychological issues, and/or are already guilty of neglect and abuse.
Texas and Florida had by far the most deaths with 142 and 110, and the hot South and South West had the most per capita.
Children can't regulate their temperature as efficiently as adults and their body temperatures warm up three to five times faster.
California law imposes a $100 fine on anyone who lives a child under six in a car 'where there are conditions that present a significant risk to the child's health or safety'.
'Never leave your child alone in a car even for a second to dash into a store or to run a quick errand,' police said.
'Summer routines can shift unexpectedly. Even the best parents can forget a quiet or sleeping child is in the backseat of a car.'
Comment: For hours?
Police can also level various charges from neglect to manslaughter depending on what harm comes to the child.
However, parents who mistakenly believe their children were not in the car are often not charged as the pain of losing a child is considered punishment enough.
Some prosecutors still charge parents with manslaughter and many have gone to jail.
As difficult as it is to imagine forgetting your child in a car, experts explain that parents often function on autopilot and lose awareness that a child is there.
Comment: One would presume this is more likely when the child is not biologically yours, because the physiologically changes, and bonding, are much less likely to be in effect.
Statistics show wide variability in race, socioeconomics, and occupations of parents who fatally left their children in vehicles, suggesting it can happen to anyone.
Cases were rare before the 1990s when parents were advised to put young children in the back seat as their airbag could kill them in an accident.
But this had the unintended consequence of making children strapped into car seats much easier to forget, and even more with rear-facing seats for infants.
Comment: Is this really the reason? Because there are a plethora of other issues now plaguing the US, associated with societal dysfunction, and that have been on the rise since the 1990s, and so perhaps these avoidable deaths are yet another sign of a societal deterioration as a whole.
No Heat Stroke suggested putting a soft toy in the front seat to remind you there was a child in the car, or putting items you need in the back seat next to the child.
Vehicles should also be locked at all times and keys kept out of reach to stop curious children climbing in and getting locked inside.
'Teach children that vehicles are never to be used as a play area, and that if they can't get out of the rear doors, to try the front doors and failing that, to honk the horn to get the attention of others,' it said.
If this not criminal negligence then I don't know what is....were they attending a 'pride' parade ??