Last Aug. 8, Mark Warschauer drove to his office, parked his car, locked it and walked away—forgetting that his 10-month-old son Mikey was in the backseat. When Warschauer returned three hours later, Mikey was dead. The heat of the closed car had killed him.

There is no name anyone can call Warschauer, 50, vice chairman of the education department of the University of California at Irvine, that he hasn't called himself. He could not be more heartbroken. As hard as it is to believe a parent could ever simply forget about a child in a car, Warschauer has been cleared of any wrongdoing in his son's death. "There was nothing to indicate he was ever a bad father," says Orange County deputy district attorney Matt Murphy, who investigated the case for two months. "This was an incredibly tragic accident."

Cases like Warschauer's are rare but not unheard of: Last year 42 children in the U.S. died after being left in hot cars, a record. Warschauer and his wife, Keiko Hirata, 40, a political science research fellow at U.C. Irvine who recently gave birth to second son Daniel Kaito, born with Down syndrome, discussed their experience with PEOPLE's Lyndon Stambler.

Warschauer: We had tried so hard to have Mikey. It took four years and three rounds of in-vitro. We were enormously proud and happy. We baby-proofed the house to the ultimate degree. We checked on him constantly at night.

Hirata: The week before Aug. 8 was very busy and tiring. Mikey had been sick earlier in the week, and I knew he was very tired that morning. I thought, Okay, he can take a nap at daycare.

Warschauer: I took him at the door, and I put him in the car seat. Usually Mikey made lots of sounds: "Da, da, da. Ma, ma, ma, ma." But that morning he must have fallen asleep. I must have been absorbed in thinking about work. Sometimes I go on automatic pilot as I concentrate on ideas. People always saw that as a strength. Of course it doesn't look like such a strength anymore. I was not mindful enough that morning.

Hirata: With office-related things or keys, Mark sometimes might forget. But with Mikey he was extremely careful.

Warschauer: At about 11:30 I went out for lunch. While I was walking back, I saw a crowd in the parking lot. I wondered what was going on.

I saw a baby lying on the ground. I got closer, and I saw it was Mikey. I was shocked. Then I got closer still, and I saw that the window on my car had been smashed in. That's when it all came together for me: "Oh my God, I must have left Mikey in the car!" The police said, "He's dead." It was like a knife in my heart. I screamed. I just collapsed on the ground, sobbing.

The police didn't arrest me, but they took me to the station. Once I started to become more aware, I was worried about Keiko. I think the police drove me home around 3 or 4. I was so ashamed and afraid Keiko would never want to see me again. I was overwhelmed with her love and support. Of course I blame myself, and I will my entire life. Keiko was so devastated.

Hirata: I never blamed Mark. I knew from the bottom of my heart how much Mark loved Mikey. When Mikey died, I blamed myself too. I decided that Mikey should go to the daycare that morning. He could have stayed here. I couldn't go on by myself, without Mark. I needed my husband at my side.

For the first two months I used to hold Mikey's pajamas before I went to sleep. Now I go to the cemetery almost every day. I still think about the accident, and I ask myself questions like, Did he suffer? Did he cry? Every time I get into a warm car I think about him.

Warschauer: We couldn't eat. I lost about 20 lbs. Keiko probably lost 15 lbs. I got a few nasty e-mails, but other than that, everyone was really supportive. Still, even to this day it's hard to look people in the eye as much as I did.

Hirata: I cried a lot. I just couldn't get out of bed. Going to a psychiatrist helped tremendously. He confirmed that even good people can make tragic mistakes.

Warschauer: I would say the coping began from the very beginning. We spent a lot of time together. We'd go to beautiful places. We'd take walks. We'd talk about Mikey, about the meaning of life, about what do we do now.

A few months after the tragedy, the couple decided to go ahead with the plans they had made before Mikey's death to have another child. When blood tests indicated the baby was at high risk for Down syndrome, they refused an amnio.

Hirata: I wanted to be a mother and give life. I was so obsessed with death. I didn't want to commit suicide. But I didn't know that I had the courage to live. When I found out I was pregnant, I felt, I can't die. I have to protect this life.

Warschauer: Our feelings about the pregnancy were mixed with so much sadness about Mikey. Then there were worries: Will we be able to be good parents? Will we be overprotective?

I don't know if this accident could happen to anyone, but through some of the groups we've been involved in we've met other good people who have lost a child to hyperthermia. I hope that people reading this will realize how easily something like this can happen. I hope they'll take some basic safety precautions. Put a teddy bear or diaper bag in the front seat as a reminder every time your child is in the car seat. And make sure your daycare provider calls you if your child doesn't show up.

Hirata: I'm grieving more intensely now about Mikey than before. I wish he was here so that he could protect his little brother.