WASHINGTON — The Justice Department has released a newly declassified version of a May 2004 legal memo approving the National Security Agency’s Stellarwind program, a set of warrantless surveillance and data collection activities that President George W. Bush secretly authorized after the terrorist attacks of Sept. 11, 2001. But questions about the program remain.

A more heavily redacted version of the memo had been released in 2011 as part of Freedom of Information Act lawsuits by the American Civil Liberties Union and the Electronic Privacy Information Center. The new version includes previously censored references to the existence of the data collection related to Americans’ phone calls and emails.

The Obama administration voluntarily reprocessed the memo from Jack Goldsmith, the head of the Justice Department’s Office of Legal Counsel, in light of the fact that it had declassified the existence of the bulk phone and email data programs last year after leaks by Edward J. Snowden, a former N.S.A. contractor.

The fuller release adds to the public record of an important historical episode. However, the government continued to redact crucial portions of the memo that would answer a primary remaining question about the history of Stellarwind: What prompted the Justice Department to conclude in early 2004 that one aspect of the program, which collected records about Americans’ emails in bulk, was illegal — even though it permitted other aspects, like warrantless wiretapping and the bulk collection of Americans’ phone records, to continue?