This task is about writing a correct answer for the reading comprehension task. Based on the information provided in a given passage, you should identify the shortest continuous text span from the passage that serves as an answer to the given question. Avoid answers that are incorrect or provides incomplete justification for the question.

Passage: From 1877 to 1878, the area suffered a rash of stagecoach robberies. Most notable, however, were the two holdups committed by masked road-agent William Whitney Brazelton. Brazelton held up two stages in the summer of 1878 near Point of Mountain Station approximately 17 mi (27 km) northwest of Tucson. John Clum, of Tombstone, Arizona fame was one of the passengers. Brazelton was eventually tracked down and killed on Monday August 19, 1878, in a mesquite bosque along the Santa Cruz River 3 miles (5 km) south of Tucson by Pima County Sheriff Charles A. Shibell and his citizen's posse. Brazelton had been suspected of highway robbery not only in the Tucson area, but also in the Prescott region and Silver City, New Mexico area as well. Brazelton's crimes prompted John J. Valentine, Sr. of Wells, Fargo & Co. to send special agent and future Pima County sheriff Bob Paul to investigate. Fort Lowell, then east of Tucson, was established to help protect settlers from Apache attacks. In 1882, Frank Stilwell was implicated in the murder of Morgan Earp by Cowboy Pete Spence's wife, Marietta, at the coroner's inquest on Morgan Earp's shooting. The coroner's jury concluded that Spence, Stilwell, Frederick Bode, and Florentino "Indian Charlie" Cruz were the prime suspects in the assassination of Morgan Earp. :250 Deputy U.S. Marshal Wyatt Earp gathered a few trusted friends and accompanied Virgil Earp and his family as they traveled to Benson for a train ride to California. They found Stilwell lying in wait for Virgil in the Tucson station and killed him on the tracks. After killing Stilwell, Wyatt deputized others and rode on a vendetta, killing three more cowboys over the next few days before leaving the state. Question: Who was held responsible for the murder of Morgan Earp?
Frank Stilwell