Some of these were British subjects, while the remainder claimed American citizenship.

The former were charged with high treason, the penalty for which is death.

Those claiming to be aliens, and citizens of the United States, were indicted under an old statute which was enacted during the period of the Canadian Rebellion of 1837, which provided that subjects of a foreign state who entered Canada for the purpose of levying war rendered themselves liable, on conviction, to the death penalty.
On the 26th of July, 1866, President Andrew Johnson sent to the United States Congress the following documents from the Department of State, in reply to two resolutions of the House of Representatives, the first requesting him to urge upon the Canadian authorities, and also upon the British Government, the release of the Fenian prisoners captured in Canada; and the second requesting him to cause the prosecutions instituted in the United States against the Fenians to be discontinued, if not incompatible with the public interest:-- DEPARTMENT OF STATE, WASHINGTON, July 26, 1866.

To the President:-- The Secretary of State, to whom was referred two resolutions of the House of Representatives, passed on the 23rd of July, instant, in the following words, respectively:-- "Resolved, that the House of Representatives respectfully request the President of the United States to urge upon the Canadian authorities, and also the British Government, the release of the Fenian prisoners recently captured in Canada.
"Resolved, that the House respectfully request the President to cause the prosecutions instituted in the United States courts against the Fenians, to be discontinued, if compatible with the public interests." Has the honor to report, in regard to the first resolution, that the Government of the United States holds no correspondence directly upon any subject with the Canadian authorities mentioned in the said resolution, or with the authorities of any colony, province or dependency of any other sovereign state; and that, on the contrary, all its correspondence concerning questions which arise in, or effect, or relate to such colonies, provinces or dependencies, is always conducted exclusively with such foreign governments.
On the 11th of June last a note was addressed by this department to the Honorable Sir Frederick W.A.Bruce, Her Majesty's Minister Plenipotentiary residing in the United States, of which a copy is hereunto annexed.

It is proper to say, in relation to that note, first, that the reports mentioned therein to the effect that prisoners had been taken on the soil of the United States and conveyed to Canada, and threatened by Canadian agents with immediate execution without legal trial, were found on examination to be untrue and without foundation in fact.