As demonstrations have evolved with the help of text messages and online social networks, so too has the response of law enforcement.

On Thursday, F.B.I. agents descended on a house in Jackson Heights, Queens, and spent 16 hours searching it. The most likely reason for the raid: a man who lived there had helped coordinate communications among protesters at the Group of 20 summit in Pittsburgh.

The man, Elliot Madison, 41, a social worker who has described himself as an anarchist, had been arrested in Pittsburgh on Sept. 24 and charged with hindering apprehension or prosecution, criminal use of a communication facility and possession of instruments of crime. The Pennsylvania State Police said he was found in a hotel room with computers and police scanners while using the social-networking site Twitter to spread information about police movements. He has denied wrongdoing.

American protesters first made widespread use of mass text messages in New York, during the 2004 Republican National Convention, when hundreds of people used a system called TXTmob to share information. Messages, sent as events unfolded, allowed demonstrators and others to react quickly to word of arrests, police mobilizations and roving rallies. Mass texting has since become a valued tool among protesters, particularly at large-scale demonstrations.