There is a certain sliding over and indeterminateness in describing the beginning of the firing. Major Pitcairn who was good man in a bad cause, insisted upon it to the day of his death, that the colonist fired first…He does not say that he saw the colonists fire first. Had he said it, I would have believed him, being a man of integrity and honor. He expressly says he did not see who fired first; and yet believed the peasants began. His account is this—that riding up to them he ordered them to disperse; which they not doing instantly, he turned about to order his troops to draw out as to surround and disarm them. As he turned, he saw a gun in a peasants hand from behind a wall, flash in the pan without going off; and instantly or very soon two or three guns went off by which he found his horse wounded and also a man near him wounded. These guns he did not see, but believing they could not come from his own people, doubted not and so asserted that they came from our people and that thus they began the attack. The impetuosity of the King’s troops were such that promiscuous, uncommanded but general fire took place, which Pitcairn could not prevent; though he struck his staff or sword downwards with all earnestness as a signal to forbear or cease firing. This account Major Pitcairn himself gave Mr. Brown of Providence who was seized with flour and carried to Boston a few days after the battle; and Governor Sessions told me.
From the diary of Ezra Stiles, president of Yale College, entry for August 21, 1775.