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

Joshua Wolf, 24, could remain behind bars until next summer, when the grand jury investigating the incident is due to expire.

Wolf had sold footage of the protest to San Francisco television stations and posted it on his Web site. Investigators are seeking portions of his videotape that haven't been broadcast.

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 footage may reveal who was behind the melee, part of an anarchist-led protest over the G-8 international economic conference last year in Scotland. A San Francisco police officer also was injured.

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

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 as U.S. marshals removed Wolf from the courtroom.

Wolf's lawyer, Jose Luis Fuentes, said that relinquishing the footage to a grand jury would be tantamount to his client 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federal authorities are disregarding California's shield law, which generally allows journalists to decline to divulge unpublished material to state authorities.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 it 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.