Alison's Attempted Killer is a previously anonymous character in Pretty Little Liars, who attempted to kill Alison DiLaurentis by hitting her on the head with a rock during the Liars' sleepover on September 1, 2009.
In Season 5, Alison's killer is revealed to also be Big A. In "A is for Answers", the person who tried to kill Alison and the person who buried her are two different people. Jessica DiLaurentis saw who hit Alison with a rock, and subsequently buried her daughter alive, believing she was dead. In "Game Over, Charles", it was revealed that Charlotte hit Alison, mistaking her for Bethany who was going to hurt Jessica.
At first, the Liars suspected Toby Cavanaugh to be Alison's killer, since he was blamed for "The Jenna Thing", giving him a motive. He also has a "9/01 Free at Last" tattoo, which was thought to be memorializing the day Alison disappeared. This was actually the day he was free of Jenna.
Toby was once again suspected by the Rosewood Police Department after they found out that it was Toby's sweater Alison wore the night she disappeared. Toby planned to leave town, as he would be convicted for Alison's murder - whether he is guilty or not. The police discover his whereabouts and arrested him for the murder of Alison DiLaurentis. He was cleared of his arrest after the blood evidence was corrupted.
The girls also suspected Melissa's boyfriend, Ian Thomas, who had an affair with Alison. His name was found carved on a tree along with Alison's and 'A' sent the Liars a video of Ian and Alison at The Kissing Rock the night she disappeared. The Liars told the police about their suspicions, but were ignored due to Ian's good reputation. Emily later found the N.A.T. Club videos which Alison had stolen, suggesting Ian had killed her because she knew about them.
At the dance, a drunk Emily walks up to Ian and warns him they all know what he did and that he won't get away with it. Ian pulled Spencer apart and calmly asked her to explain. She lied and said that it was because she told her friends they kissed that summer.
The girls remember when Alison had sneaked them into a frat party with fake IDs. Ian was in attendance too, tipsy and flirting with a girl who wasn't Melissa. Later at the party, that same girl falls down the staircase and the liars see her unconscious body lying on the floor. When Emily looked up, she saw Ian standing at the top watching the scene looking eerily calm.
In the episode, they find the trophy from Hilton Head, which they think belonged to Ian. There's a giant stain on the pedestal and they assume it's Alison's blood and this is what she was murdered with. They quickly decide to hand it over to the cops.
The next day at school the liars are approached by officer Barry. He is not amused and asks them whether they planned this. The girls aren't comprehending. Gloating, he explains to them the trophy is a fake and the blood on the pedestal comes from a rat. The girls will have to come with him for questioning, again. As they walk up to the police car, Ian is watching with an insidious smile.
When Spencer is looking for Melissa's cell phone in the church, Ian attempts to kill her, learning that Spencer was the one leaving anonymous ultimatums on his phone. While there, he seems to admit to killing Alison and tells Spencer he's going to have to do the same to her.
In an attempt to escape, Spencer throws away the only piece of evidence linked to Ian: the flash drive, with Ian's "footage." Ian also presumably died that night.
Wren and Melissa are trying to help Ian by giving him drugs. However, when they arrive at the place where he is hiding: Melissa, Wren, and the Liars are all shocked to find Ian dead once and for all. He has a bloody shot to his head, a gun in his hand, and a letter lying on the ground that when the camera quickly pans over it, reads, "I killed Alison, I lost my temper because she knew too much. But there is only so much you can bury. It won't be that easy, but I know how to get rid of the pain. I can't run from the law. Come and find me. -Ian." It's later revealed that the scene was faked by 'A'. The Liars also saw footage of Ian leaving Alison at the Kissing Rock, clearing him of suspicion.
Alison's brother, Jason, was suspected by the Liars in Season 2 as he woke up the morning after Alison's disappearance with a note in his pocket saying "I know what you did". He was also suspected at the beginning of Season 5, as Mrs. DiLaurentis must have known Alison's attacker if she protected them, leading the Liars to think Jason hit Alison with the rock.
Garrett Reynolds was also suspected of Alison's murder, due to the fact that he stole page five of Alison's autopsy report. He was subsequently arrested and charged with both Alison and Maya's murders. His innocence was proven when his blood was not found on the Alison's anklet and when Lyndon James was revealed as Maya's stalker and murderer. Jenna, however, thought he killed Alison as he pretended to with Spencer's old field hockey stick.
Cece tells Emily that Wilden was at Cape May the summer before Alison disappeared. The girls then suspect Detective Darren Wilden, as it was revealed that he may have had a sexual relationship with Alison the summer she disappeared in Cape May. CeCe Drake supported this theory as she believed Wilden got Alison pregnant and tried to kill her to cover it up.
It was revealed that Alison's attempted killer was her older sister (later revealed to be her cousin), Charlotte, otherwise known as CeCe Drake. Charlotte thought it was Bethany who escaped from Radley wearing her clothes and was planning on hurting Jessica. Jessica buried Alison, thinking she was dead and to protect her adoptive daughter, while CeCe cried nearby begging her mom to forgive her. After that, Jessica paid Wilden to take Charlotte to Radley Sanitarium and to lie about that night.