WASHINGTON (AP) — The U.S. Sentencing Commission says the advent of the Internet has led to broad sentencing disparities in child pornography cases, and that the existing penalty structure needs to be changed.

In a study for Congress, the commission said Wednesday that federal law enforcement agencies handle nearly 2,000 child pornography prosecutions annually, up from 700 a decade ago. It attributed that increase largely to pornographers exploiting the Internet.

The report says the current sentencing scheme fails to distinguish adequately among offenders based on how dangerous they are, and is overly severe in some cases.

The commission urged Congress to amend the law to take into account the volume of an offender's child pornography collection, the types of sexual conduct depicted in the images, the ages of the victims depicted, how long an offender has maintained a collection and how involved an offender is with others in an Internet "community" devoted to child pornography.

One sign that the approach to sentencing needs to be changed is that judges are more frequently disregarding the sentencing guideline ranges for child pornography. In 2004, 84 percent of sentences for producers of pornography were within the guideline range. By 2011, only half of the sentences were within the range. That suggested there were unduly harsh sentences for some offenders and unduly lenient sentences for others, the commission said.

Average federal prison sentences for defendants convicted of child pornography offenses have almost doubled in the past decade to five years for possession of pornography and 11 years for distribution.

When it comes to pornography over the Internet, "child pornography offenders engage in a variety of behaviors reflecting different degrees of culpability and sexual dangerousness that are not currently accounted for in the guidelines," said U.S. District Judge Patti B. Saris, the sentencing commission's chairwoman.

Advances in Internet and computer technologies have resulted in the growth of Internet-based child pornography communities, which not only operate as a forum for offenders to receive and distribute images but also serve to validate and normalize the sexual exploitation of children, the study said.

"Such communities thrive in Internet chat rooms and bulletin board systems and also through the use of ... file-sharing programs in which offenders directly communicate with each other," the study added. The commission's data suggests that a significant minority of offenders — about one-fourth — have some level of involvement in such forums.

The study also found that one in three child pornography offenders had a known history of engaging in illegal sexual misconduct before federal prosecutors brought them to justice. The commission based its conclusion on a review of nearly 2,700 child pornography cases.

The commission also said 30 percent of child pornography offenders were arrested again after serving their sentences, a figure that is in line with the federal offender population generally. The commission says only a small proportion — about 7 percent — were arrested for sexual misconduct after serving their sentences.