Two 16-year-old twins did everything together. They got straight A's in school, took dance classes, and according to police, killed together.

Inside a suburban home in Conyers, Georgia, a horrifying crime scene so bloody, it looks like a slaughterhouse. Slumped in the bathtub, the naked body of a middle-aged woman.

Police are baffled. Who would want Nikki Whitehead, loving and devoted single mom, dead?

Nikki Whitehead was literally born in prison, her mother doing time for drug possession.

Nikki was raised by her grandmother, who had a tough time keeping her on the straight and narrow. At only 17 years old, Nikki ended up pregnant with twins, naming the girls Tasmiyah and Jasmiyah Whitehead, known as Tas and Jas.

For 12 years the trio lived with Nikki's grandmother, the girls' great-grandmother. But Nikki dreamed of giving her daughters a better life. So Nikki and the twins moved about 20 miles away, to Conyers, Georgia. The young mom made sure her girls had the advantages that eluded her, putting them in dance and music classes. The girls were straight-A students with hopes of one day going to Harvard.

But their path to success was dramatically diverted when the girls hit high school. Tas and Jas bitterly resented their mother's discipline. And soon the rage would boil over, when Nikki took away Tas's cellphone. It was clear that Nikki was losing control.

Nikki made a frantic 911 call when she discovered Jas had snuck out in the middle of the night to meet her boyfriend. Two once-sweet girls had suddenly morphed into twin terrors.

Conyers Police Officer Myra Scruggs responded to a call from Nikki after yet another blowout. At the time, the twins told the officer they didn't want to live with their mom and insisted on moving back with their great-grandmother, where the rules were relaxed.

But after interviewing Nikki, Tas and Jas, Officer Scruggs says she had an unsettling feeling.

"The girls seemed very innocent, very sweet, but the look in Nikki's face, she was fearful of those children. She knew that they worked together," said Officer Scruggs.

After a few hours, everything appeared to have calmed down.

"The girls were just ready to, 'OK we're going to go to our rooms and we're going to go to bed and maybe we'll just call our great-gradma tomorrow and we can go see her," said Scruggs. "And they left it like it was all OK."

But Officer Scruggs had a gnawing feeling that something just wasn't right. Instead of leaving the subdivision, she stayed close.

"I just knew it wasn't over," said Scruggs. "Within I would say three or four minutes, I could hear screaming coming from the direction of the house. Both of them jumped on her and began to beat her."

Nikki manages to get away, and calls 911. Officer Scruggs rushed back to the house.

"Nikki is running out of the house with a cordless phone in her hand, hysterical," said Scruggs. "I pulled up, asked her what happened. She said as soon as I left, that the girls had attacked her."

Tas and Jas tell an entirely different story.

"The twins said that once they went back in the house after we left, that their mother became abusive with them, told them they were not going anywhere, and started to beat them," said Scruggs.

Scruggs said the physical evidence clearly showed that Nikki was the victim.

"She had scratches on her neck, scratches on her chest, and she was hysterical," said Officer Scruggs. "The girls, on the other hand, had no marks on them, no indication that really they had been in any kind of altercation. Talking with them, I didn't believe a thing that the girls had said. You could have been two strangers and there would have been more emotion shown."

This time Tas and Jas are taken into custody and charged. They are in and out of court for years, and ordered, along with their mother, to undergo counseling.

Family members knew the twins resented Nikki, but most chalked it up to a rebellious teenage phase.

But these mother-daughter spats are nothing compared to the next time the cops are called to the house, on January 13, 2010.

The girls are in an absolute panic. The twins tell police they came home from school to find their mother dead, lying in a blood-soaked bathtub.

Police say Nikki Whitehead was stabbed 80 times with a kitchen knife, her spinal cord nearly severed.

Nikki and her daughters had a history of arguments and trouble, but it was unfathomable to think these two petite 16-year-olds were capable of murder.

Immediately after the horrendous discovery, the twins are consoled by police, who are simply looking for answers. Who would want Nikki dead?

Cops soon learn the girls' father has an ironclad alibi: He was living in Canada at the time of the murder.

Then the girls themselves tell investigators that Nikki has two boyfriends. Was a love triangle what sparked the murder?

"She lived with 'Robert,' and she also had a boyfriend named 'Joe,'" said Conyers Police Capt. Jackie Dunn. "The twins had indicated that Robert had overheard her speaking to the other boyfriend on the phone, and indicated that Robert had confronted Nikki about the infidelity."

But as cops are tracking leads, and long before detectives had Tas and Jas in a quiet interrogation room, they noticed something odd on the drive to headquarters.

"One of the twins we observed in the back of the ambulance, we saw her bite her arm," said Dunn. "We stopped her and asked her, 'Hey, what are you doing?' and she explained that she does that when she gets upset."

In a quiet interrogation room, a startling comment catches Detective Ken Swift off-guard.

"These two girls were hugging each other in each other's arms, and when I said, 'What can I do to make this easier for you?' They turned and looked at me and they said 'Can we watch "CSI"?'" said Det. Swift. "And immediately the hairs on the back of my neck stood straight up. Essentially it was right then that it was like OK, this is -- something was very, very off."

Police notice something even more suspicious: the girls' complete lack of empathy for their dead mother.

"They would cry a lot, but it was tearless crying," said Conyers Police Capt. Jackie Dunn. "When I would ask them to describe their mother, they were very insulting of their mother."

During the interview, detectives observe the twins are still wearing gloves. Asked about a scratch, one of the twins says she got into a fight with the other. As they weave their story to cops, back at the bloody crime scene investigators uncovered physical evidence yielding valuable clues -- and in Tas's bedroom, what could be the most macabre piece of the murder puzzle.

"I notice there's a pair of brown boots that have a significant amount of blood on them, and they are in a shoebox," said Det. Swift. "I found a clump of hair, which normally a clump of hair is insignificant in a female's house simply because there is hair everywhere, but when that clump of hair is pulled out of somebody's head, and then has been wrapped in a napkin and stuffed into the toe of a shoe, you begin to realize that the twins were not telling us anything that was truthful."

Investigators begin to realize it's no former boyfriend or husband who carried out this murder. The killers are right under their noses.

The twins' lies begin to unravel further when detectives retrace their movements on the day of the murder. The twins tell police they missed the bus to school and had to walk. They told police they left at 7:30 a.m. But surveillance video from a nearby gas station shows the twins did not walk to school that day. They actually hitched a ride from a stranger.

"We began catching them in lies concerning their time frames or their details," said Swift. "They began to be very angry and would become argumentative and very uncooperative."

And the high school surveillance video shows the twins in the hallway hours after they told police they arrived at school.

"We were able to determine that they arrived there at mid-morning, where they told us they were only about 10 minutes late for school," said Capt. Dunn.

Detectives separate the teens, putting them in different interrogation rooms. Being apart seems to wear them down.

But with only circumstantial evidence prior to testing what's found in the home, cops can't hold the girls, who are released to their great-grandmother. For the next four months Tas and Jas lead a normal life, going to school, hanging out with friends, even attending prom. Meanwhile police are watching, waiting and testing the evidence, quietly building their case.

"We found items in their closet that had blood on them and so when we we're trying to distinguish whose blood, you cannot determine between twins because they have the same DNA," said Capt. Dunn.

And then a startling clue, almost as if it came from beyond the grave.

"To Nikki's credit, she did not go down without a fight, and she left evidence on one of her girls as a marker," said Swift.

Earlier, police saw Tas biting her arm, and she explained it away as a nervous habit. Detectives say she was trying to cover up Nikki's bite mark on her arm.

"We had a dental examination of Nikki's teeth, and they matched it to the bite on the twin," said Dunn.

And autopsy results revealed another grim detail: "They found human hair between Nikki's teeth," said Dunn.

"We would surmise that twin was behind Nikki and had her from around the front, and Nikki is fighting for her life and bit her arm," said Dunn. "And that's how the hair became embedded in her teeth."

Cops are quickly zeroing in on what appear to be two pretty little liars. And on the last day of school before summer break, with enough probable cause, detectives arrest Tas at her home. Police go to Rockdale High to nab her sister Jas.

"We decided we were going to arrest them on the last day of school for fear of they were going to take flight out of the country to avoid the charges," said Dunn.

The twins are charged with felony murder and aggravated assault in their mother's death.

Police didn't quite understand what the motive was, but they were about to hear a truly shocking story.

The identical twin sisters are caught in a web of lies and incriminating evidence.

The 16-year-old girls are hauled off to jail, keeping silent about the chilling crime.

But after nearly four years awaiting trial, the twins finally come forward with a stunning admission, confessing to the brutal attack and murder on tape.

Jas: "I think I picked up a knife and I stabbed her. ... I think I stabbed her in the stomach. ... It was multiple times."

Tas: "I think I had her hands, and Jas, her feet. It's kind of like a joint effort. She was heavy. We just put her in [the tub]. The water was turned on."

Detective: "At some point in time that morning, you did realize that morning your mom was dead."

Tas: "Yeah."

As part of a deal with the state to avoid trial, Tas and Jas pleaded guilty to voluntary manslaughter in April. Both were sentenced to 30 years in prison.

But the question remains: What really happened that dark day?

"By their account they were up till 2 or 3 o'clock in the morning," said Capt. Dunn. "They didn't want to get up when they were awakened to go to school by their mother, and I think that's what sparked the attack, the confrontation. And that they killed their mother, and then they spent a couple of hours fabricating parts of the crime scene, and cleaning it up, and then they went to school so that they would have an alibi."

Detectives say they found a journal in the home that revealed a premeditated plan to murder their mother for some time.

"They would write back and forth to each other how much they disliked their mother and even to the point that they said in their writings that if they didn't get rid of her soon, they weren't going to be able to," said Dunn.

"This was an act of rage. The two girls had planned for this event," said Det. Swift.

Cops believe Nikki never stood a chance in the savage attack. The bloody path even extends to outside the home, suggesting Nikki managed to get away, but the girls dragged her back to the house to finish her off.

"At some point Nikki escaped, made it to the house next door," said Dunn. "When we interviewed that resident, he recalled someone yelling and he heard someone ring his doorbell. He was in bed at the time and he didn't get up to investigate. We could see where Nikki's bare feet exited and re-entered through that door in blood. So we would surmise that she was pulled back into that house, where the attack resumed."

Back in the house, the heinous assault continued. The girls smashed their mother over the head with a pot, stabbed her with a kitchen knife and choked her with the ribbon of a medal. Then they dragged her body into the tub, where they left her to die.

In the recording of the confession, a detective asks Jas what Nikki said when she was in the tub.

Jas: "'Kill me now or I'll kill y'all.'"

Det.: "What happened then?"

Jas: "She went under a couple of times and that was it."

"I'm sure that their confession was self-interested," said Dunn. "They were looking out for their selves and had been coached by attorneys, and they said enough to live up to their part of the plea."

By the time the twins serve their 30-year sentences, they will be in their 40s. In the meantime, Jas has traded in her jailhouse jumpsuit for a different uniform, at least for a day: She graduated high school in the slammer, and even served as class valedictorian.