A federal appeals court in Denver has ruled that e-mailed images obtained by the National Center for Missing and Exploited Children constituted a warrantless search and therefore must be suppressed as part of a child pornography case.

The 10th Circuit Court of Appeals ruled last Friday in favor of a Kansas man who sent an e-mail in April 2013 with four attachments that included suspected child porn via his AOL account. AOL immediately flagged the message via its hash value matching algorithm, believing one of the attached images was suspect, and sent them all on to NCMEC. (Providers have a “duty to report” to the NCMEC if their users access, transmit, or store child pornography.) The agency then opened his message and confirmed that Walter Ackerman had indeed attempted to transmit not just one, but four illegal images.

The following month, a Homeland Security Investigations special agent got the tip through the NCMEC system, and he sought and received a warrant to search Ackerman’s home in Lebanon, Kansas. Under questioning, Ackerman admitted to distributing child pornography via e-mail. Months later, Ackerman was formally indicted on two counts of child pornography. His lawyers filed a motion to suppress in February 2014, arguing that his e-mail was searched illegally. Ackerman eventually accepted a plea deal in September 2014. Although he was sentenced to 170 months in prison, he was kept out of custody pending an appeal on the Fourth Amendment question.

Read more: ARSTechnica