"I just see how hollow my life is now," says Dennis. "And I never thought I would experience that in my lifetime."
"I believe the reason I was put on this Earth was to have Amy," says Mary.
Before the accident, the Hills appeared to be living the extremely good life outside Orlando, Fla. They owned a successful marketing business, lived in a stately home, and their daughters, Amy and Kaitlin, were excelling in and out of school.
On Aug. 7, 2000, Mary and Dennis went to pick their daughter Amy up from her first day of eighth grade. With her were best friends Carrie Brown and Zak Rockwell. The three were inseparable.
On the way home, Dennis told Mary he needed to make a stop. Mary drove off with the three kids in the back seat. Everything would change during the next few minutes. Correspondent Peter Van Sant reports on how this seven-mile trip home from school ended in tragedy.
"I've driven that street thousands of times, every day," says Mary. "You come out of a turn and you're just starting to accelerate and you kind of feel, you know, your car. I just felt like the rear end wasn't with me and I immediately started to stop. And it wouldn't."
Although Mary says she was pumping the brakes, the car kept accelerating: "I have been a good driver my whole life. But I could not stop that car. My last thought was 'Why won't you stop?'"
Investigators believe that Mary's BMW was traveling at 73 miles an hour when it slammed into a tree. The point of impact left a two-foot divot, right next to where Amy was sitting. Her best friend, Carrie, was in the middle with Zak, the farthest from the point of impact. Mary, who was not wearing her seatbelt, was thrown 24 feet out the side window.
"I woke up. I thought I was on fire," says Mary. "The heat was immense."
"It was just the most horrible experience I could ever imagine," says Dennis, one of the first people to arrive on the scene. He immediately went to the back seat and tried to feel the pulse of his daughter. "I thought, 'Oh please, I'm not doing this right. I'm doing something wrong. She's still alive and I just can't tell.'"
Dennis then checked for Carrie's pulse and felt nothing. Zak was unconscious but breathing.
"I just could not believe this was happening. I wanted to say, 'OK guys. Cut. Let's start this over. Let's play this scene over,' because this is not the way I want it to be," says Dennis. "Just five minutes before, they were alive. Life was good. And then, five minutes later, your life changes like you never thought that it could."
A 48 Hours animation shows how police believe the accident happened. After Mary made a left turn, her car began to accelerate rapidly. As it started to drift into the oncoming traffic lane, it fishtailed and began a sideways slide.
Police estimated the car was traveling 73 miles an hour when it left the road. After it hit the tree, the car traveled 20 feet to its resting spot on the other side of the tree.
Zak's dad, Keith Rockwell, assumed his son was taking the bus home that day, until police showed up at his door. He rushed to the hospital to find his 13-year-old son alive, but unconscious. "They basically said he was in a coma, and that the longer he was in, the worse it would be," recalls Rockwell.
Carrie's mother, Rita Brown, was running errands when a friend called her cell phone and insisted on meeting her. "I collapsed. I couldn't believe it. Not my baby. Not my baby," says Brown.
But once some of the shock started to wear off, the questions started. Was this tragic crash really accidental?
For Mary, the guilt of driving the car that killed the two girls, and left the third child in a coma, was unbearable: "Why couldn't I have died? I lived a life, you know? I cannot accept something that I don't have a reason for. I am angry and everyone is angry, I know, at me."
Rockwell was terrified, but considered himself strangely lucky that his son was alive: "As a single dad, it's just me and him, and he's No. 1."
For four days, Zak's loved ones kept a bedside vigil until he woke up. Although Zak has no memory of the accident, he was on the road to recovery. But some wounds would never heal. "I lost a lifelong friend, partner," says Zak. "I lost a lot of things."
"I felt sorrow and anger. I was mad," says Brown, a world-renowned Olympic gymnastic coach. "I was just mad that Carrie was gone and it wasn't fair. Why my baby? Why is she gone? Why did this happen?"
But Brown's grief soon turned to anger when she learned the details of what one witness saw just before the crash.
Jimmy Arthur was on his way home from work when he saw Hill's BMW. "She almost sideswiped me. I mean, she just came out from no place," says Arthur, who was attempting to change lanes when he first encountered Mary's BMW. "I didn't feel like there was any mechanical problem. It just seemed like she was upset or angry or just in a very, very big hurry."
She was in such a hurry, adds Arthur, that she overshot a red light: "She put the car in reverse and quickly backed up, burning the tires and I thought she was going to hit the front of the van at that time. Once the light changed, she floored it, and the tires were screaming and she took off."
Arthur says he watched as Mary then lost control of her car and wrapped it around the tree. And his eyewitness account would become crucial to investigators as details about Mary began to surface.
"In a speeding car, I feel unsafe. And I usually did in Mrs. Hill's car," says Zak. "She always seemed like she was depressed."
In fact, Mary had been receiving treatment for depression. And in the days leading up to the accident, investigators learned Mary had been behaving strangely, including making a bizarre phone call to Brown.
"She says, 'Well, I'm going in Monday for shock treatment' and I said, 'Shock treatment for what?' And she goes, 'Well, I haven't been doing well and they thought they might try that,'" recalls Brown, who wasn't the only witness to Mary's unraveling.
Vicki Hartzell encountered Mary one afternoon in her driveway. "She was weaving and she was very unsteady on her feet," recalls Hartzell.
Hartzell says she told her two daughters that they weren't allowed to ride in the car with Mary anymore. "I felt there was something wrong with her," she says.
But a blood test after the crash revealed that Mary wasn't using drugs or alcohol. So, did Mary lose control of her car or her emotions?
On the advice of her attorneys, Mary did not speak to the victims' families. "To tell you the truth, there aren't any words in the English language that could ever express my sorrow for what happened," says Mary. "'I'm sorry what happened to your daughter.' I can't face that woman and say that to her. Everything I have known is gone for me -- my work, my life, my mobility, my spirit. I don't know what else to be taken. I don't know how it could be any worse."
Eight months after the accident, when Mary was at rock bottom, prosecutors charged Hill with two counts of vehicular homicide and manslaughter. If convicted, she could be sent to prison for 30 years. "I never expected it," says Mary. "Not for the most fleeting second could I think that someone would think that I would do something like that intentionally."
Now, Mary and Dennis Hill are fighting back. "We've endured our share, and it's time to turn the tables now. I believe that we're gonna be ready," says Dennis, who has hired high-priced defense attorney Gerald Boyle, best known for defending killer Jeffrey Dahmer, to prove his wife did not intentionally kill their daughter and her friend.
"Mary was an exceptional driver," says Dennis. "I never quite understood how this could possibly happen."
But defense experts think they have an answer. After examining the BMW, they believe a fault in the BMW's cruise control caused the car to speed up on its own.
"Once the investigators were able to say 'Hey there's a problem with the cruise control here. The throttle linkage,'" says Dennis. "Well then, pieces of the puzzle started to go together."
BMW owner Larry Gustafson has never met Mary, but he knows what it's like to have a car suddenly race off. "The first time it happened to me, I had no idea what was going on," says Gustafson. "It went from 35 to 40 miles an hour to almost 60 miles an hour, just that quick."
Gustafson claims his mechanic fixed the problem on his used BMW by replacing an electrical circuit. But Carrie's mother isn't buying it.
"Mary Hill needs to be held accountable for her actions. And if that includes going to jail, then so be it. The judge and jury will decide," says Brown, who believes that Mary is to blame for Carrie's death.
Both mothers are unable to share their grief over the loss of their daughters, Amy and Carrie, for the battle lines have been drawn.
Mary, charged with vehicular homicide and manslaughter, has become a recluse.
"It's like a jail. I look out and see the fence and keeping out the outside world," says Mary. "It's very empty. It's like a mausoleum. It is as quiet as a tomb. It became unbearable. For over two years, I wouldn't leave this home."
Making things difficult were the rumors flying furiously through their posh Florida community -- rumors that Mary had a history of depression, drug use, and dangerous driving.
Sworn statements by Deane David, the Hills' former nanny, fueled those rumors. "I know that something like this was going to happen," says David. "She was a horrible mother. … That was my worst fear, was her in the car with them."
"She has accused me of horrible things," says Mary of Dean, who worked for the Hills for less than year, and left after a falling out over pay six months before the accident.
David's most scathing allegation was that Mary abused cocaine. "Two weeks into the job, I found out that Mary had a cocaine addiction," says David. "Her purse had fallen off the counter, and in that, a compact had fallen out with powder on the mirror and a razor blade."
But Mary denies these allegations: "I have tried it. I will not say I haven't tried it. But no, I never had an addiction. I don't think trying it once makes you an addict."