In c. 800, Abd al-Malik was also placed as tutor over Harun's son al-Qasim. His prominence and influence with the army made Harun distrust him and in 803 he was arrested and thrown into prison. The actual reason remains unclear, although most sources agree that his own son, Abd ar-Rahman, informed the Caliph that he was allegedly planning to overthrow him. Abd al-Malik remained imprisoned until Harun's death six years later, when the Caliph al-Amin (r. 809–813) released him. Al-Amin's succession was contested by his elder half-brother al-Ma'mun, and there was unrest in Syria. Abd al-Malik still wielded considerable influence over the frontier troops, and therefore he was appointed governor of Syria and Upper Mesopotamia and tasked with securing these regions for al-Amin and raising troops to confront al-Ma'mun. Soon after reaching his seat at Raqqa, however, Abd al-Malik fell ill and died. His grave was demolished a few years later by the victorious al-Ma'mun, allegedly because Abd al-Malik had sworn never to accept al-Ma'mun's rule.