But there are fewer precedents explaining how courts should evaluate criminal subpoenas, according to legal experts. Perhaps that is because prosecutors are more cautious about the risk of violating the First Amendment and so issue fewer criminal subpoenas, or because the subpoenas themselves carry language prohibiting disclosure of their terms.

“In the criminal context it’s trickier because it’s the government asking for stuff, and I think it’s going to be harder to fashion a rule, especially when the government is not exactly willing to part with the reasons” for requesting the information in the first place, said Jonathan Zittrain, a law professor at Harvard.

Without knowing the motives of prosecutors, he continued, judges may be hard-pressed to balance their needs against the importance of free speech.

Bloggers concerned about possible litigation may want to check the privacy policies of their Internet service providers, to see whether they include a pledge to notify any customer whose site is the subject of a subpoena, Mr. Zittrain said.

Armed with that knowledge, a blogger could fight the subpoena in court. Software also exists that is intended to make it difficult to identify those who want to be anonymous online.

Image Ben Smith, a co-founder of Room 8. Credit... Fred R. Conrad/The New York Times

Some of the people blogging on the Room 8 site are named, but many choose to be anonymous. Mr. Smith said he called the assistant district attorney in the Bronx who had issued the subpoena to try to find out more about why prosecutors wanted the Internet Protocol, or I.P. address, of the person who blogged under the name Republican Dissident. But the prosecutors would not share any information, he said.