WASHINGTON (AP) — The closed-circuit video of Timothy McVeigh's execution is expected to be a prime target of activists — from death penalty opponents who might want to prove the sentence is cruel to paramilitary groups who might want to show McVeigh as a martyr.

The Justice Department won't say much about the measures it is taking to ensure that the pictures of the May 16 execution are not public. Security experts said Thursday it would be difficult, but not impossible to intercept and decode the video.

Attorney General John Ashcroft kept his description of the transmission methods vague when speaking to reporters.

"The broadcast will use the latest encryption technology integrated with state-of-the-art video-conferencing over high-speed digital telephone lines," Ashcroft said. "Federal regulations prohibit any recording of the execution. Therefore, any closed-circuit transmission will be instantaneous and contemporaneous."

Security experts say Ashcroft described ISDN, short for Integrated Services Digital Network. ISDN lines can transmit data at least twice as fast as normal telephone lines, although the data travel through public phone networks.

Because of the distance between the execution site in Terre Haute, Ind., and the victims' families in Oklahoma, the most traditional and secure method of closed-circuit video — laying a straight cable between the two points — is not practical.

McVeigh was convicted of the April 1995 bombing of the federal building in Oklahoma City that left 168 people dead, including 19 children. He is scheduled to be put to death by lethal injection at a federal prison facility on May 16.

President Clinton's grand jury testimony in the Monica Lewinsky case was carried from the White House to a federal courthouse about a mile away through a single cable which the public had no access to. ISDN brings different challenges for would-be hackers, said Mark Rasch, a former Justice Department and security expert.

"You have to get physical access to either the network or the line. If you can get physical access to it, you can literally splice in," Rasch said.

That's where the encryption comes in. Even if hackers successfully tapped a line between the Indiana prison and Oklahoma, they still would have to decrypt the signal in order to get a usable image.

One advantage for hackers is that they could save the encrypted video, then take all the time they wanted to decrypt it, Rasch said.

"If you hack it and store that feed, you don't have to do it right away. You could do it in a week, a month, a year, or 10 years. In 10 years, who knows what the force of the attacks might be," he said. "It would be difficult, but not impossible."

Recording the execution is prohibited by law, although Rasch noted that even sending the images to Oklahoma is technically recording it for a short time.

"They're taking a very narrow interpretation of recording," he said of the Justice Department's decision. "They say they're not recording, they're broadcasting. They could broadcast it on CNN, and that's not a recording. Just that nobody could hit 'record' on their VCR."

This is not the first time, even in the McVeigh case, that courts have used this method of videoconferencing. Washington-based Polysonics Corp. broadcast McVeigh's trial in Denver to Oklahoma City, and had to overcome the same security obstacles.

Polysonics President George Spano said he could not talk about the trial without getting clearance from a judge, but said his company will not be transmitting the execution.

Bruce Schneier, a noted cryptography expert and author of several books on code-making techniques, was interviewed by McVeigh's attorneys in connection with the broadcast of the Denver trial. He said that several companies — or even the military — could provide practically hack-proof encryption for the signal, and that a more likely security leak is a low-tech approach.

"The real risk is some audience member with a tape recorder or a little handheld camera who wants to make a bunch of money by selling the tape to a TV station," Schneier said. "Cameras are teeny now."