Twenty years ago, as Rodney King was beaten by Los Angeles police officers, a private citizen in a nearby apartment turned on his video camera. Largely because of that tape, four officers were criminally charged. In July, a homeless schizophrenic man died after a police beating in Fullerton, Calif. Audio from a cellphone video caught Kelly Thomas’ cries for his father and helped force an investigation that resulted in a first-degree murder charge against one police officer.

The increasing availability of cellphones and video cameras has fundamentally changed police abuse cases, creating vital evidence in cases that were once dismissed as matters of conflicting accounts between officers and citizens. With that change, however, has come a backlash from officers who, despite court rulings upholding the right of citizens to tape police in public, have been threatening or arresting people for the “crime” of recording them. In many states, prosecutors have fought to support such claims and put citizens in jail for videotaping officers, even in cases of police abuse.

In New York this year, Emily Good was arrested after videotaping the arrest of a man at a traffic stop in Rochester. Good was filming from her front yard; an officer is heard saying to her, “I don’t feel safe with you standing behind me, so I’m going to ask you to go into your house.” When she continued to film, the officer said, “You seem very anti-police,” and arrested her.

In Illinois last month, Brad Williams filed a lawsuit against the Chicago Police Department because, he said, he was beaten by police in response to his filming an officer holding and dragging a man down the street from inside a moving squad car. Ironically, Chicago has rejected complaints about the installation of thousands of cameras in the city that film citizens in public for use in prosecutions.

In Maryland in July, Anthony Graber got a well-deserved speeding ticket, but his real mistake was posting footage from his motorcycle helmet-cam on YouTube. It showed an irate off-duty, out-of-uniform officer pulling him over with his gun drawn. Prosecutors obtained a grand jury indictment against Graber on felony wiretap charges, which carry a 16-year prison sentence.

In Boston in August, the 1st U.S. Circuit Court of Appeals ruled unambiguously that the Constitution protects citizen videographers filming in public. In that case, attorney Simon Glik was walking past the Boston Common on Oct. 1, 2007, when he came upon three Boston officers arresting a man. Glik turned on his cellphone camera after hearing a witness say the police were being abusive. An officer told Glik to turn off his camera. When Glik refused, he was arrested for violation of the state wiretap statute, disturbing the peace and, for good measure, aiding in the escape of a prisoner.

The charges were dismissed after a public outcry, but in a later civil rights case, city attorneys fought to deny citizens the right to videotape police. The court rejected Boston’s arguments and found that the police had denied Glik his First and Fourth Amendment rights.

But other federal judges might not be so sure. Take Richard Posner, the intellectual leader of conservative judges and scholars who sits on the U.S. Court of Appeals in Chicago. Posner shocked many last month when he cut off an attorney for the American Civil Liberties Union, which had filed suit to challenge an Illinois law preventing audio recording of police without their consent.

The ACLU lawyer had uttered just 14 words when Posner barked: “I’m not interested, really, in what you want to do with these recordings of peoples’ encounters with the police.” Posner then added his concerns about meddling citizens: “Once all this stuff can be recorded, there’s going to be a lot more of this snooping around by reporters and bloggers. ... I’m always suspicious when the civil liberties people start telling the police how to do their business.”

Many judges may privately share Posner’s view of such confrontations. And the near-total silence of politicians in dealing with the question of the public’s right to record what they see and hear suggests that many legislators may also find these cases inconvenient.

Actions against citizen videographers run against not just the Constitution but good public policy. Yet, without a videotape, Rodney King would have been just another guy with a prior record claiming abuse, against the word of multiple officers.

The outcome once was all but inevitable: no tape, no case. As long as police abuse is out of sight, it can also be out of mind. If successful, the backlash against citizens recording police could guarantee that Rodney King is never repeated — the officers’ trial, that is.

• Turley is a professor of public interest law at George Washington University.