In fact its authenticity would never have been impugned at all had it not (wrongly) blamed Cresap with killing Logan's family.
Cresap's defenders, with curious folly, have in consequence thought it necessary to show, not that Logan was mistaken, but that he never delivered the speech at all.
The truth seems to be that Cresap, without provocation, but after being incited to war by Conolly's letter, murdered some peaceful Indians, among whom there were certainly some friends and possibly some relations of Logan (see testimony of Col.

Ebenezer Zane, in Jefferson's Notes, and "American Pioneer," I., 12; also Clark's letter in the Jefferson Papers); but that he had no share in the massacre of Logan's family at Yellow Creek by Greathouse and his crew two or three days afterwards.

The two massacres occurring so near together, however, produced the impression not only among the Indians but among many whites (as shown in the body of this work), that Cresap had been guilty of both; and this Logan undoubtedly believed, as can be seen by the letter he wrote and left tied to a war club in a murdered settler's house.

This was an injustice to Cresap; but it was a very natural mistake on Logan's part.
After the speech was recited it attracted much attention; was published in newspapers, periodicals, etc., and was extensively quoted.

Jefferson, as we learn from his Papers at Washington, took it down in 1775, getting it from Lord Dunmore's officers, and published it in his "Notes," in 1784; unfortunately he took for granted that its allegations as regards Cresap were true, and accordingly prefaced it by a very unjust attack on the reputed murderer.