From the Washington Post December 15, 2007
http://www.washingtonpost.com/wp-dyn/content/article/2007/12/14/AR2007121402257.html?nav=rss_technology/techpolicy
Many defendants obsessively collect huge numbers of images -- the FBI recently seized a computer with 1.5 million images suspected to be child pornography -- and many have molested children, officials say. People who download the images once or twice, possibly by accident, are unlikely to be prosecuted, officials said.
"You can't wrap your brain around what we're talking about here," said Bonnie S. Greenberg, a prosecutor in the U.S. attorney's office in Maryland. "We're not talking about a 16-year-old who looks like she could be 19. We're seeing prepubescent children who are being raped, babies, toddlers being tied up."
This sounds to me the "believe" part comes down to "they HAD TO know".....