By Meade Jorgenson
According to the National Center for Missing and Exploited Children (NCMEC), there are about 500,000 convicted sex offenders across the country who do what they're supposed to do: register themselves with the local police in their communities.
But, according to NCMEC, about a 1 in 5 convicted sex offenders do not register themselves. And local authorities don't always have the manpower or resources to go looking for every one of them. In response, the U.S. Marshals Service started a program called, "Operation Guardian," which has targeted the worst of the worst -- approximately 450 of the most dangerous sex offenders who remain on the run.
The U.S. Marshals invited Dateline NBC to accompany one of their arrest teams, based in Maryland and led by Inspector Roger Wilson and Inspector Tom McDaniel.
Watch this special web-only edition of 'The Hansen Files' below:
U.S. Marshals use social media to help in the search for a violent sex offender on the run from authorities in Pennsylvania. Dateline NBC's Chris Hansen reports this web exclusive edition of 'The Hansen Files.'