Who poisoned Baha'u'llah?

Baha'u'llah himself attempted to poison his rival (and his own half-brother) Subh-i-Azal. We have an account of this event from multiple sources:

"It was in fact the Mírzá [i.e. Baha’u’llah] who had sought to poison His Holiness [Azal]. [On the day in question] Sultán Khánúm (entitled the Greatest Holy Leaf), who at the time was still a believer in her most august uncle [i.e. Azal], with blinks and glances, [when the food was brought out by the servants] indicated that that he [Azal] should not partake of the meal placed before him. When the Mírzá [i.e. Bahá’u’llah] insisted that His Holiness should eat, His Holiness refused, thereupon the aforementioned saw that His Holiness had become appraised of their evil intentions [i.e. to kill him], and therefore, in order to deflect attention [from the deed], he proceeded to eat a few portions from that plate of food himself. Immediately his [i.e. Baha’u’llah’s] state was overturned [i.e. he became violently ill]. He then threw the plate of food in the courtyard outside. In that yard there was a dog who immediately began to eat [the discarded food] and thereupon fell dead shortly thereafter. When the Mírzá beheld this scene, he was immediately embarrassed, and so sought out a physician. After regaining his health, he proceeded to spread rumours that His Holiness [Azal] had attempted to poison him." (The True Acccount of Takur of Nur, p. 43-44).

"The first juggle and trick of sorcery which he [i.e. Baha] outlined was this, that he brought to Hazrat-i Azal [i.e. Subh-i Azal] a dish of plain food, with one side of which he had mixed with some poison, intending to poison His Holiness. For hitherto the apportioned breakfast and supper of His Holiness the Fruit [Hazrat-i Thamara, one of the titles conferred on Subh-i Azal by the Point] had been from the house of Mirza Husayn Ali [i.e. Baha]. When that poisoned dish was placed before His Holiness, Mirza Husayn Ali pressed him to partake of it. By a fortunate chance the smell of onions was perceptible in the food, and His Holiness, being averse to taste it. Mirza Husayn Ali continued to press him urgently to eat. He replied, “it smells of onions, I will not eat it; if it is so good, eat it yourself. From this answer Mirza Husayn Ali supposed that His Holiness had divined his evil design, and, simply put the view of disguising the truth and putting a better appearance on the matter, ate a little from the other side [i.e. the unpoisoned side] of the dish, in order that the suspicion of His Holiness might perhaps be dispelled and he might eat the poisoned side. But His Holiness, because of the smell of onion would not eat." (Hasht Bihisht, p. 304-305).