Sometimes you come across an image online that’s so horrifying you can’t unsee it. For Krzysztof Franaszek, it happened at work.

Franaszek runs the advertising research firm Adalytics based in the US. Recently, he was studying where ads for the US Department of Homeland Security end up online, and the project took him to an image-sharing website called ImgBB. There, Franaszek uncovered something sickening: sexually explicit images of a very young child, with adverts for Fortune 500 companies running alongside them.

He immediately reported the content to the US Federal Bureau of Investigation (FBI), the Department of Homeland Security (DHS), and child safety organisations. The Canadian Centre for Child Protection – one of those Franaszek alerted – says it found at least 35 images flagged by Adalytics on the site that meet its classification of child sexual abuse material (CSAM). The Centre says it notified ImgBB, and the images were taken down. An FBI spokesperson says the bureau reviews all allegations of criminal conduct but does not comment on tips from the public. The DHS did not respond to questions.

The more Franaszek dug, the clearer the problem became – and his findings raise questions about how the adverts you see online may also be inadvertently pumping large sums of money into undesirable, and at times illegal, corners of the internet.