The Library of Congress’ online Civil War photograph collection (https://www.loc.gov/pictures/collection/cwp/about.html) provides access to about 7,000 different views and portraits made during the American Civil War (1861-1865) and its immediate aftermath. The images represent the original glass plate negatives made under the supervision of Mathew Brady and Alexander Gardner as well as the photographic prints in the Civil War photographs file in the Prints & Photographs Reading Room. These negatives and prints are sometimes referred to as the Anthony-Taylor-Rand-Ordway-Eaton Collection to indicate the previous owners. The Library purchased the negatives in 1943.
As a publicly supported institution, the LOC generally does not own the rights to materials in their collections. Users should determine for themselves whether or not an item is protected by copyright or in the public domain, and then satisfy any copyright or use restrictions when publishing or distributing materials from our collections. Transmission or reproduction of protected items beyond what is allowed by fair use or other exemptions requires written permission from the copyright holder (https://www.loc.gov/legal/).
The site provides an excellent overview of copyright usage (https://www.loc.gov/legal/understanding-copyright/).