Last week the Tumblr app was mysteriously yanked from Apple’s App Store–and now we know why. The site was found to be hosting child pornography images that slipped past the company’s filters, reports CNET’s Download.com .

Standard practice at Tumblr is to scan every image a user uploads to the site against a national database of known child pornography images. But it appears that new, unknown images of child pornography were uploaded to Tumblr sometime last week or earlier that weren’t flagged as such.

Apple has a strict “inappropriate content” clause in its App Store policies that says the company will immediately remove apps that are found to be hosting illegal content. Tumblr’s disappearance so quickly from the App Store shows just how swiftly Apple acts on such breaches to its policies.

As for Tumblr, the company told Download.com today:

“We’re committed to helping build a safe online environment for all users, and we have a zero tolerance policy when it comes to media featuring child sexual exploitation and abuse. As this is an industry-wide problem, we work collaboratively with our industry peers and partners like the National Center for Missing and Exploited Children (NCMEC) to actively monitor content uploaded to the platform. “Every image uploaded to Tumblr is scanned against an industry database of known child sexual abuse material, and images that are detected never reach the platform. A routine audit discovered content on our platform that had not yet been included in the industry database. We immediately removed this content. Content safeguards are a challenging aspect of operating scaled platforms. We’re continuously assessing further steps we can take to improve and there is no higher priority for our team.”

Tumblr is currently working with Apple to get the app reinstated in the App Store now that the illegal images have been pulled. It should be noted that this is not the first time Apple has pulled an app from the App Store due to it hosting illegal content. Early this year, Apple pulled the popular secure messaging app Telegram from the App Store because it was being used to share child pornography.