Morse arrived in the evening of August 3 and slept in the guest room that night. After breakfast the next morning, at which Andrew, Abby, Lizzie, Morse and Sullivan were present, Andrew and Morse went to the sitting room, where they chatted for nearly an hour. Morse left around 8:48 am to buy a pair of oxen and visit his niece in Fall River, planning to return to the Borden home for lunch at noon.[23] Andrew left for his morning walk sometime after 9 am.[24]

Although the cleaning of the guest room was one of Lizzie and Emma's regular chores, Abby went upstairs sometime between 9:00 am and 10:30 am to make the bed.[25] According to the forensic investigation, Abby was facing her killer at the time of the attack.[26] She was first struck on the side of the head with a hatchet, which cut her just above the ear, causing her to turn and fall face down on the floor, creating contusions on her nose and forehead.[27] Her killer then struck her multiple times, delivering seventeen more direct hits to the back of her head, killing her.[26]

When Andrew returned at around 10:30 am, his key failed to open the door, so he knocked. Sullivan went to unlock the door; finding it jammed, she uttered a curse.[28] She would later testify that she heard Lizzie laughing immediately after this; she did not see Lizzie, but stated that the laughter was coming from the top of the stairs.[28] This was considered significant as Abby was already dead by this time, and her body would have been visible to anyone on the home's second floor.[28] Lizzie later denied being upstairs and testified that her father had asked her where Abby was, to which she replied that a messenger had delivered Abby a summons to visit a sick friend.[29]