Jeremy Belknap, whose History of New Hampshire was written some years before its publication, and who was a contemporary and friend of Sullivan and Langdon, gives us the following account, which, in spite of criticisms upon it, will bear the closest scrutiny, and is essentially accurate as are most of the accounts of that first historian of New Hampshire.
Belknap’s History of New Hampshire (1812), Vol, II, p. 288:
An order having been passed by the King in Council prohibiting the exportation of gunpowder and other military stores to America, a copy of it was brought by express to Portsmouth at a time when a ship of war was daily expected from Boston with a party of troops to take possession of Fort William and Mary, at the entrance of the harbor. The committee of the town with all possible secrecy and dispatch collected a company from that and some of the neighboring towns ; and before the Governor had any suspicion of their intentions, they proceeded to Newcastle and assaulted the Fort. The Captain and his five men (which was the whole garrison) were confined, and one hundred barrels of powder were carried off. The next day another company went and removed fifteen of the lightest cannon, and all the small arms, with some other warlike stores, which they distributed in the several towns under the care of the committees. Major John Sullivan and Captain John Langdon distinguished themselves as leaders in this affair. It was transacted with great expedition and alacrity, and in the most fortunate point of time, just before the arrival of the Scarboro [sic] frigate and Canseau sloop, with several companies of soldiers, who took possession of the Fort and of the heavy cannon which had not been removed.
The governor put the five men who belonged to the fort on board the ship of war to be reserved as evidences in case of a prosecution of the offenders for high treason ; and having consulted council in this and the neighboring Province, thought it his duty ; that he might prevent any charge of misprision of treason against himself; to dismiss from public trust, all those persons concerned in the assault of the fort, who had held any office under the government and concerning whose proceedings he had authentic testimony. He also issued a proclamation, commanding all officers, civil and military, to assist in detecting and securing the offenders and exhorting all the people to beware of being seduced by the false arts and menaces of abandoned men.