County authorities armed with a search warrant rifled desks and cabinets in a television station newsroom here today, seizing two videotapes filmed during two days of rioting at the Idaho State Penitentiary this week.

The search was the second of its kind since the U.S. Supreme Court ruled May 31, 1978, that newsrooms could be searched for evidence. The only other court-authorized search of a newsroom since that decision was last May, when the Flint Voice, a Michigan weekly, was invaded by police attempting to learn the source of a leaked document critical of the mayor, according to Jack Landau, director of the Reporters Committee for Freedom of the Press.

Officials of the station, KBCI-TV, a CBS affiliate, said that only a 15-minute segment of the tapes had been aired. News director Paul Reiss said much of the material was obtained in confidence and he feared that the station's credibility in protecting the anonymity of news sources had been severely damaged.

"When I saw them going through my desks, I saw my freedom going away," said Rob Loy, the station's managing editor.

Last Wednesday night, while inmates held two guards hostage, prisoners asked for Loy by name. Loy, with a cameraman, was permitted inside the prison compound to hear the inmates' grievances. The KBCI camera rolled 30 minutes as the two toured the facility with the inmates, the only reporters to do so until officials regained control of the yard, without resistance, at dawn Thursday.

Ada County prosecuting attorney Jim C. Harris said the tapes, especially the sequence shot behind the prison gates by Loy's cameraman, were the "best available evidence" to establish the identity of inmates who took part in the violent uprising.

Harris was critical of station management for not turning over the tapes voluntarily. He said he shared the "frustration" of other law enforcement authorities "in having not only to confront criminals at the Idaho State Penitentiary for purposes of investigation but also having to battle representatives of the news media in that regard."

Lawyers for the station said they had known of the existence of the warrant since Thursday afternoon, the day prison officials recaptured the burned-out pentientiary.

William J. Russell, the station's Boise Attorney, said no effort was made to remove the tapes from the building out of fear that the KBCI staff would be charged with concealing evidence or obstructing justice.

"We though we were going to have enough time to fight this in the courts," Russell said at the studio today. He said Harris told him the warrant would not be executed for a week.

While the search was in progress, two sheriff's deputies were stationed 10 blocks away at the law offices of Elam & Burke, the station's counsel. One of the attorneys representing the station, Ryan P. Armbruster, said he was detained for 25 minutes until the officers were called off.

Harris said the decision to secure the law offices was made after authorities learned that the tapes at the studios were copies of the original footage. Authorities believed the originals might have been hidden in the law offices, and guards were posted while officials conferred on whether to seek a second search warrant.

Harris said he decided against searching the law office when the station's attorneys pledged they would not remove the originals from Ada County.

The search began at 9:05 a.m. when H. D. Pfeiffer, an investigator in Harris' office, knocked on the station's door.He was accompanied by five Ada County sheriff's deputies and a former Boise television newsman, John Rooney, who is now the official spokesman for the Idaho Department of Law Enforcement.

After Pfeiffer read the warrant demanding all tapes filmed at the prison Wednesday and Thursday, Riess said, "I feel this is a clear infringement of our First Amendment rights. But seeing how we have no legal recourse, I suppose we have to let you search our building."

Pfeiffer asked where the films were located. Reiss replied, "I think you see where the newsroom is."

A videotape made by a KBCI cameraman of the deputies going through personal notes in the newsroom also was briefly seized.

"We feel we've been raped," Loy said as the deputies left the building with only the two prison tapes in hand.

The new media generally prefer to deal with subpoenas, rather than search warrants, for evidence. A subpoena, a court order requiring the production of documents or testimony, can be opposed in court, giving the judge the right to decide if the evidence is necessary after both sides are heard.

Harris said he sought a search warrant because Idaho law bars a subpoena in cases where a defendant has not yet been named. He also said he needed to act swiftly because some prisoners were being transported out of state.

Harris said he may seek search warrants for Boise's other two commercial television stations and the city's newspaper, the Idaho Statesman.

KBCI, meanwhile, has made no decision on its next step, but its lawyers have not ruled out the possibility of a lawsuit against county officials.