SAN FRANCISCO -- A freelance video journalist was jailed Tuesday for refusing to give a grand jury his unpublished footage from a July 2005 protest in which anarchists were suspected of vandalizing a police car.

Joshua Wolf, 24, sold the footage to San Francisco television stations and posted it on his Web site. He could remain behind bars until next summer, when the grand jury investigating the incident is due to expire. A San Francisco police officer was also injured in the melee.

U.S. District Judge William Alsup said there is no federal law shielding journalists from participating in grand jury investigations. The judge sided with prosecutors who suspect the unpublished material may reveal who was behind the incident, part of an anarchist-led protest over the G-8 international economic conference last year in Scotland.

"This is direct evidence of what happened," Alsup said.

As marshals went to remove Wolf from the courtroom, Alsup said he wasn't jailing Wolf to punish him. "The purpose of this is to get you to change your mind," the judge said.

Wolf's lawyer, Jose Luis Fuentes, said for Wolf to turn over the unpublished information would amount to him becoming "an arm of the government." Because of the subpoena, Fuentes said the underground groups Wolf chronicles are denying him access.

The American Civil Liberties Union said federal authorities are disregarding California's shield law, which generally allows journalists to decline to divulge unpublished material to state authorities. That shield, however, does not attach to federal investigations.

Although the incident involved the San Francisco police, federal authorities are investigating because the incident involves the destruction of federally funded property.

"We're taking the position that the government hasn't shown it has a connection to a legitimate federal interest here," ACLU attorney Alan Schlosser said after the two-hour hearing.

Wolf's jailing comes as federal authorities also seek to compel the testimony of two San Francisco Chronicle reporters who obtained the grand jury testimony of Barry Bonds, Jason Giambi and other participants in the government's steroids investigation.

Lance Williams and Mark Fainaru-Wada have challenged a subpoena ordering them to divulge who leaked them transcripts from the secret grand jury proceedings. They are due to appear Friday before U.S. District Judge Jeffrey White for a hearing on their bid to quash the subpoena.

Williams and Fainaru-Wada published a series of stories in 2004, and this year published a book, all based largely on secret testimony from the Bay Area Laboratory Co-Operative steroids probe.

The government, in both the Wolf and Chronicle cases, repeatedly cites a 1972 decision by the U.S. Supreme Court. In that case, Branzburg v. Hayes, Justice Byron White ruled that reporters, like everyone else, must "respond to relevant questions put to them in the course of a valid grand jury investigation or criminal trial."

The same case was successfully invoked in the investigation of who leaked the name of CIA agent Valerie Plame. Then-New York Times reporter Judith Miller spent 85 days in jail last year for refusing to testify in that case, which led to perjury and obstruction of justice charges against Vice President Dick Cheney's top aide, Lewis I. "Scooter" Libby.

The Chronicle reporters, like Wolf, are arguing that the First Amendment right of speech is more important than what the grand jury may gain from their appearance. The Chronicle reporters say they will also go to jail rather than reveal their source or sources.