Director of Civil Liberties at the Center for Internet and Society Jennifer Stisa Granick spoke with Christina Ng of ABC News to discuss the legality of the NYPD's plans to subpoena user information from Twitter in the interest of halting an attack.

The New York Police Department has demanded that Twitter release the name of a user who threatened an attack "just like in Aurora" on the Broadway theater where Mike Tyson's one-man show is playing.

The NYPD plans to subpoena Twitter today for the user's identity after the social media giant refused authorities' emergency request for the information.

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If Twitter were to turn over the user's identity at the first request, it could be liable for any mistake or potential invasion of privacy, according to Jennifer Granick, the director of civil liberties for Stanford Law School's Center for Internet and Society.

"The law prohibits providers from turning certain information over voluntarily and, if they do, they can be sued," Granick said. "But the government can compel the information from the provider with varying degrees of legal process depending on what the information is. When it's the name associated with the account, the government can get that with just the subpoena."

The federal law is part of the Electronic Communications Privacy Act, passed in 1986. There are exceptions to the law, Granick said. Exceptions can be made if there's a threat of serious bodily injury or death. But that first decision is up to the provider.