The Proposition

Set in the Australian outback in the 1880s, the movie follows a series of events following the horrific rape and murder of the Hopkins family, likely committed by the infamous Burns brothers gang.
The film opens in a remote wood building with a violent gunfight between the police and Charlie Burns' (Guy Pearce) gang, which ends with the deaths of all of the gang members except for Charlie and his younger brother Mikey. Captain Stanley (Ray Winstone) makes a proposition to Charlie: he and the feeble-minded Mikey can go free of the crimes they have committed if Charlie kills his older brother, Arthur (Danny Huston). Arthur is a mercurial psychopath who is so vicious the Aboriginal tribes refer to him as "The Dog Man" and both the police and the Aborigines refuse to go near his camp. Captain Stanley states his intention to civilize the harsh Australian wilderness by bringing Arthur to justice and using Mikey as leverage. Charlie has nine days to find and kill Arthur, or else Mikey will be hanged from the gallows on Christmas Day.
Captain Stanley's motivations for taming Australia are revealed: he has been forced to move there with his delicate wife, Martha Stanley (Emily Watson), and apparently wants to make it a safer place for them to live. The Stanleys were friends of the Hopkins family, leading Martha to have nightmares about her dead friends. Word spreads of Stanley's deal with Charlie, primarily from Stanley's corrupt subordinate, Sergeant Lawrence (Robert Morgan), causing disgust among the townspeople.
Shortly thereafter, Eden Fletcher (David Wenham), for whom Captain Stanley works, orders that Mikey be given one hundred lashes as punishment for the rape and murder of the Hopkins family. Stanley is aghast at this, not only because he believes Mikey is likely innocent and the flogging may kill him or harm him irreparably, but also because it will break his deal with Charlie and bring the Burns gang's revenge upon him and his wife. Stanley sends Sergeant Lawrence away with tracker Jacko (David Gulpillil) and other men to "investigate" the reported slaying of Dan O'Riley by a group of Aborigines.
Meanwhile, Charlie rides in search of Arthur, drinking and apparently reflecting. Along the way, he encounters an inebriated old man named Jellon Lamb (John Hurt). In the course of conversation, Charlie realizes that Lamb is a bounty hunter in pursuit of the Burns brothers and knocks him out. Later on, after sleeping on a rock bed, Charlie awakes and is speared in the chest by a group of Aboriginal men standing over him. Seconds later a gunshot is heard and the head of the man who threw the spear explodes. Charlie then passes out.
Charlie wakes up in the camp of his brother Arthur, located in caves among desolate mountains. Arthur's gang consists of Samuel Stoat (Tom Budge), who shot the Aboriginal man who had speared Charlie; a woman named Queenie (Leah Purcell) who tends to Charlie's wound; and a muscular Aboriginal man called Two-Bob (Tom E. Lewis). As he recovers from his wounds, Charlie has several opportunities to kill his brother, but does not.
Captain Stanley attempts to defend Mikey by gunpoint from the bloodthirsty townspeople, but is overruled once Martha arrives, insisting on revenge for her dead friends. Mikey is brutally flogged and horrifically wounded. The formerly excited townspeople slowly become disgusted and Martha faints at the ghastly display. Captain Stanley grabs the whip and throws it at Fletcher, staining him with blood. Fletcher fires Stanley.
Near Arthur's camp, Sergeant Lawrence and his men have found and butchered a group of Aborigines. Arthur and Two-Bob find Lawrence's group while they sleep and kill Jacko and Sergeant Lawrence. Before Arthur stomps Lawrence to death, Lawrence tells Arthur that Charlie has been sent to kill him. Jellon Lamb enters Arthur's camp and ties up Samuel and Charlie, both of whom are sleeping. Lamb is shot from behind by the returning Two-Bob. Arthur then begins torturing the still-living Lamb with a knife. Charlie points his revolver at Arthur, but instead shoots Jellon in the head, putting him out of his misery.
Charlie decides he wants to break out Mikey and informs Arthur. Arthur, Samuel and Charlie ride into town dressed in the clothes taken from the officers Arthur and Two-Bob had killed, while Two-Bob poses as an Aborigine they have captured. Once at the jail, the men free Mikey, and Charlie and Two-Bob ride off with him. Arthur and Samuel remain to torture and slaughter the two officers inside the jail. The badly injured Mikey, who has never recovered from the flogging, dies in Charlie's arms. As they bury Mikey, Two-Bob tells Charlie that all of this is Charlie's fault: "You should never have left us."
Captain Stanley and Martha let their guard down to have a peaceful Christmas dinner. Immediately following their conclusion of grace, Arthur and Samuel shoot open the door and invade their home. Arthur pulls Captain Stanley into the other room and brutally beats him, while Samuel taunts his wife. Samuel drags Martha inside, and Arthur shoots Captain Stanley through the shoulder. As Samuel assaults Martha, Charlie walks in and informs Arthur of Mikey's death; Arthur ignores the news and encourages Charlie to listen to Samuel's beautiful singing. Charlie walks up to Samuel and shoots him point blank in the head, then shoots Arthur twice, saying afterward, "No more." Arthur staggers out of the house. Charlie tells Captain Stanley "I want to be with my brother." He leaves the house and follows a trail of blood to find Arthur seated on the ground nearby and sits down next to him. Arthur states that Charlie has finally stopped him and asks what he will do now, to no answer, and dies as his brother watches the blood red sunset of the outback.

Question: Who is reported to have slain Dan O'Riley?
Answer: A group of Aborigines.