When delegates to the Republican National Convention assembled in New York in August 2004, the streets and sidewalks near Union Square and Madison Square Garden filled with demonstrators. Police officers in helmets formed barriers by stretching orange netting across intersections. Hordes of bicyclists participated in rolling protests through nighttime streets, and helicopters hovered overhead.

These tableaus and others were described as they happened in text messages that spread from mobile phone to mobile phone in New York City and beyond. The people sending and receiving the messages were using technology, developed by an anonymous group of artists and activists called the Institute for Applied Autonomy, that allowed users to form networks and transmit messages to hundreds or thousands of telephones.

Although the service, called TXTmob, was widely used by demonstrators, reporters and possibly even police officers, little was known about its inventors. Last month, however, the New York City Law Department issued a subpoena to Tad Hirsch, a doctoral candidate at the Massachusetts Institute of Technology who wrote the code that created TXTmob.

Lawyers representing the city in lawsuits filed by hundreds of people arrested during the convention asked Mr. Hirsch to hand over voluminous records revealing the content of messages exchanged on his service and identifying people who sent and received messages. Mr. Hirsch says that some of the subpoenaed material no longer exists and that he believes he has the right to keep other information secret.