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, we learn that is person is indeed the Big A. We learn in "A is for Answers" that 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, and in an attempt to protect the attacker. In "Game Over, Charles", it was revealed that CeCe hit Alison, mistaking her for Bethany who she believed was trying to hurt Jessica.
At first, the Liars suspected Toby Cavanaugh to be Alison's killer, due to the fact that he was blamed for "The Jenna Thing", giving him a motive. Another thing that made them suspect him was his "9/01 Free at Last" tattoo, presumably memorializing the day Alison disappeared. This was later revealed to be the day he was free of Jenna.
He was suspected by the Rosewood Police Department, after they found Toby's sweater which Alison wore the night she disappeared. Toby says that he plans on leaving town, as he will be convicted for Alison's murder - whether he is guilty or not. The police discover his whereabouts and, as he was planning on meeting Emily before he left, assumes that she's the one who turned him in. It's revealed that this was Jenna, as she wanted him to stay in Rosewood. Toby quietly and defiantly responds by saying, "you can chain me to this porch, and I'd still never touch you like that again." Jenna responds by slapping him. He was cleared of his status later on when the blood evidence was corrupted.
The girls also begin to suspect Melissa's boyfriend, Ian Thomas, who the Liars suspected had 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 pulls Spencer apart and calmly asks her to explain. She evades the truth by telling him it's because she told her friends they kissed that summer. Ian gets frustrated with her and threatens that if she's lying, ”someone is going to get hurt”.
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. He has a calm demeanor and doesn't exactly look surprised or shocked by the accident. As the girl is being driven away in an ambulance Ian is seen giving his statement to a cop. After that he observes from the porch as Alison and the girls get into the police car to go home.
In the episode they find the trophy from Hilton Head, which they think belongs 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 flashdrive, with Ian's "footage." She runs throughout the church until finally Ian catches up with her, attempting to throw her over the bell tower landing. Spencer begs for Ian to let her go, with the rest of the Liars on the phone overhearing the exchange. Suddenly, a dark hooded stranger throws Ian over the landing where a rope ties around his throat hanging him off the landing. When, Hanna, Aria and Emily get to the church, they find a traumatized Spencer alone, with Ian hanging off the landing, seemingly dead. When the police arrive at the church, Ian is mysteriously missing.
Wren and Melissa are trying to help Ian by giving him drugs. However, when they arrive to 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." The scene then cuts to the top of the room, where the imprint of a missing horseshoe is visible. It's later revealed that Ian killed himself and a suicide note was left telling the police of his guilt was left by his body, although this was later proven to have been 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.
The girls start to get suspicious about Garrett Reynolds, after Aria finds out from Mike that he's involved with Jenna, who they had previously believed to be A. He 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 anklet and 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, possibly making him Beach Hottie. 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.