When Imam Riḍā (P.B.U.H.) was in Khurāsān, his moral virtues and piety was such that he even attracted and captivated his enemies besides his friends. Ma’mūn was scared of the holy Imam’s (P.B.U.H.) increasing publicity and popularity. There was a rapid spread of Shīʿism, a growth in the attachment of the populace to the holy Imam (P.B.U.H.). Ma’mūn sought to find a remedy for this difficulty and decided to martyr the holy Imam (P.B.U.H.).
It is obvious that a vicious caliph like Ma’mūn, who planned all these intrigues in order to strengthen the pillars of his government, would never act in way to officially take responsibility of the Imam Riḍā’s (P.B.U.H.) martyrdom. 
But wasn’t it true that after subjecting Imam Mūsā b. Jaʿfar (P.B.U.H.) to long imprisonment and intolerable tortures and getting him martyred, that Hārūn claimed that Imam (P.B.U.H.) had died a natural death? Wasn’t Ma’mūn the son of that father? And didn’t he realize that by taking the responsibility of the innocent bloodshed of the son of Mūsā b. Jaʿfar (P.B.U.H.), he will weaken the pillars of his government even further! 
That was why, after the martyrdom of the holy Imam (P.B.U.H.), Ma’mūn tried his best to cover the facts in order to be safe from people’s accusations.
Abā Ṣalt Harawī was questioned, “How can Ma’mūn devise and accept murdering the Imam(P.B.U.H.), although he respected Imam (P.B.U.H.) and he appointed him as his heir-apparent?”
Abā Ṣalt Harawī, while confirming the Imam’s (P.B.U.H.) murder by the hands of Ma’mūn, stated, “If the caliph respected and honored Imam (P.B.U.H.), it was because he was aware of and acknowledged the Imam’s (P.B.U.H.) superiority. If he assigned the position of heir-apparent to him, it was because he wanted to falsely portray the Imam (P.B.U.H.) as worldly in order to reduce his respect in the view of masses.”
The Imam’s (P.B.U.H.) martyrdom by the hands of Ma’mūn was another oppression added to Shīʿa history and to the Prophet’s family and was another verse added to the poetry of free-minded poets.
Imam’s (P.B.U.H.) son, Imam Jawād (P.B.U.H.) washed his father’s body and said prayer for him, but not publicly known. Imam’s body was buried in Mashhad in the company of many of his Shīʿas and followers and it is for centuries that Imam Riḍā’s (P.B.U.H.) holy shrine is the cause of blessing and honor for Iranians.