Information on sex offenders in 21 states and the District of Columbia is now available on an Internet site launched this week by the federal government.

Participation by states is voluntary and assistant Attorney General Regina Schofield (search) said all state information is expected to be posted within six months.

The site does not contain any information not already made available on the Internet by each state. But it allows someone to do one search online to determine whether an individual who has been convicted in one state has moved to another.

There are more than 500,000 registered sex offenders nationwide, the Justice Department (search) said. The recidivism rate for sex crimes is four times higher than for other offenses, according to the U.S. Bureau of Justice Statistics (search).

Privacy advocates have been wary of publishing the names of people who already have served their sentence.

But several recent high-profile abductions and killings have led to calls for widened access to the information. The most recent case was the abduction of two Idaho children, one of whom was later found dead, allegedly by a convicted sex offender.

Every state but Oregon publishes the names, photos and backgrounds of at least some people convicted of a variety of sex crimes, particularly those involving children. But different rules apply as to what information can be accessed. Oregon's information is available by telephone.

Participants in the national site are: Arizona, Colorado, Delaware, Florida, Hawaii, Idaho, Illinois, Kansas, Kentucky, Louisiana, Maryland, Nebraska, Nevada, New Jersey, Ohio, Pennsylvania, Rhode Island,Tennessee, Utah, Virginia, Washington, D.C., and Wisconsin.