The Federal Bureau of Investigation has an interesting webpage that discusses several famous cases. The cases fall into several categories including terrorism, organized crime, historical ten most wanted, and civil rights. Following a case link provides the researcher with background information, summary of the investigation, photos and, occasionally, links to documents.
For example, at the conclusion of the Greenlease kidnapping page is a link to the summary report. Some information has been redacted, but the majority of the report can be read. Another example is the Mississippi Burning case, which includes a link to over 900 pages of information.