WASHINGTON — The Obama administration first learned last November about a harrowing trove of photographs that were said to document widespread torture and executions in Syrian prisons when a State Department official viewed some of the images on a laptop belonging to an antigovernment activist, a senior official said Wednesday.

The United States did not act on the photos for the past two months, officials said, because it did not have possession of the digital files and could not establish their authenticity. Nevertheless, they said, the administration believes the photos are genuine, basing that assessment in part on the meticulous way in which the bodies in the photos were numbered.

The photographs, some of which were released this week on the eve of an international peace conference on Syria, have helped prompt the administration to heighten its demand that President Bashar al-Assad release political prisoners and allow Red Cross inspectors access to the prisons.

But it seems clear that the photos that appear to document the torture and executions will not fundamentally alter American policy, which is to push for a political settlement that will remove Mr. Assad from power but to avoid direct military intervention in the conflict.