David L. Knellinger will go to trial this month in Virginia because the government says his computer's hard drive contains pornographic pictures of children. There are two basic defenses to that incendiary charge. One is to raise questions about how the pictures got there. The other is to argue that what appear to be real children are actually just images generated by computers.

Proving either defense requires a thorough analysis of the computer's hard drive by forensic experts. But a new law makes it much harder for those experts to do their work.

In Mr. Knellinger's case, prosecutors initially agreed to provide his defense team with a copy of his hard drive so they could prepare for trial. That was on July 26, 2006.

The next day, President Bush signed the Adam Walsh Child Protection and Safety Act, named after the murdered son of John Walsh, the host of ''America's Most Wanted.'' The law contains an extraordinary provision.

It says that child pornography is so toxic it must be kept in a secure government facility. Defense experts may examine it there, the law says, but they cannot make copies.

Child pornography is contraband, the law says, like guns or drugs. Because ''every instance of viewing images of child pornography represents a renewed violation of the privacy of the victims and a repetition of their abuse,'' Congress found, the images ''should not be distributed to, or copied by, child pornography defendants or their attorneys.''

Citing the new law, the prosecutors in Mr. Knellinger's case withdrew their proposal to turn over the evidence against him. They offered instead to allow Mr. Knellinger's defense team to inspect his computer's hard drive at the Federal Bureau of Investigation's offices in Richmond, Va.

In cases around the country, defendants have attacked the law, saying it violates their rights to due process and a fair trial. They also say that it does not, even on its own terms, make much sense.

''If you were to follow the logic of it,'' said Ian N. Friedman, one of Mr. Knellinger's lawyers, ''you should not be able to present the evidence to 12 jurors.''

In September, Judge Linda R. Reade of Federal District Court in Cedar Rapids, Iowa, upheld the restrictions in the law. ''Congress adopted a reasonable measure to ensure that the child pornography used in criminal trials does not escape into the public domain,'' Judge Reade wrote.

Her decision was brief and tentative, though, and it did not seriously engage some of the main objections to the law.

The law creates, for starters, an uneven playing field. The prosecution has unlimited access to the evidence. The defense must make an appointment.

The law also prejudges the crucial question in the case. The images are presumed to be child pornography before any court has said they are.

For a defense lawyer, having to work at the F.B.I. office is cumbersome and impractical, and it may endanger the lawyer-client privilege and leave a road map to defense strategies. It certainly makes the task of examining the hard drive much more expensive.

One forensic expert in the Knellinger cases said his fee would rise to $540,000 from $135,000 because it is so much more difficult to work away from his office. The cost of transporting his machinery, which fills a truck and takes three people a week to move, would be extra.

In January, the judge in the Knellinger case, Robert E. Payne of Federal District Court in Richmond, stopped just short of striking down the law. Instead, using a little legal jujitsu, he turned it inside out.

The law requires prosecutors to provide the defense team with an ''ample opportunity for inspection, viewing and examination at a government facility.'' But Judge Payne ruled that the burden of working at a government facility itself deprived Mr. Knellinger's lawyers and experts of an ample opportunity to prepare his defense.

The only solution, he said, was to order the government to turn over a copy of the hard drive. Mr. Knellinger's experts have now examined it, though Mr. Friedman would not say what their analysis yielded or what defenses his client would rely on at trial. Judge Payne's order added, perhaps in response to Judge Reade's concern that allowing defendants ready access to the evidence against them might allow child pornography to ''escape into the public domain,'' that only Mr. Knellinger's lawyers and experts could have access to the copied hard drive. He said he would punish any unauthorized distribution or publication.

Mr. Knellinger faces a maximum sentence of 140 years, which is not bad when you consider an Arizona man named Morton R. Berger. In February, the United States Supreme Court let stand Mr. Berger's 200-year sentence for possessing 20 pornographic images of children. Mr. Berger was a married 52-year-old high school teacher with no criminal record when he was arrested.

Child pornography is, of course, unspeakably disturbing and repulsive. But if the stakes in combating it are to include three-digit prison sentences, there is reason to be skeptical about laws that revise the usual procedures and presumptions. Indeed, one way society can show that it is deadly serious about a crime is to allow those accused of it the best defense possible.



Drawing (Drawing by Harry Campbell)

