In an unusual twist to the story, the BBC says it asked Facebook for an interview about the issue, and the company asked the public broadcaster to send examples of the images it had found. When the BBC did so, it says Facebook cancelled its request for an interview, and then reported the broadcaster to the child-exploitation unit of Britain’s National Crime Agency.

Of the images that it reported to Facebook, the BBC says only 20% were removed. The social network said the others did not breach its community-standard rules.

Facebook has struggled in the past to find a way of removing offensive imagery without also taking down images that are either innocuous or serve some other purpose. It got in hot water in 2016 for repeatedly removing an iconic photo from the Vietnam War—including briefly suspending the account of the Norwegian prime minister—because it was flagged as child pornography.

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The British broadcaster says it reported over 100 images, including ones that appeared on pages and in Facebook groups that were specifically designed to appeal to men with a sexual interest in children. A number of the images were of girls who appeared to be under the age of 16 in sexualized poses.

In at least one case, the image the BBC shared with Facebook (FB) appeared to be a screenshot from a video of child sexual abuse, along with a request from the poster to share child porn. The broadcaster said that this image was one of the ones Facebook said didn’t breach its standards.

The BBC also reported five users who were convicted pedophiles because Facebook’s rules forbid convicted sex offenders from having accounts on the network. None of the profiles have been removed, the public broadcaster said on Tuesday.

At FB's request we sent them images we had reported, which weren't taken down by FB moderators. FB then reported us to the police.

— Angus Crawford (@AngusCrawfordR4) March 7, 2017

Facebook told the BBC in a statement that it had reviewed the material and “have now removed all items that were illegal or against our standards. We take this matter extremely seriously and we continue to improve our reporting and take-down measures.” The company said it followed industry practice when it reported the BBC to the Child Exploitation and Online Protection Centre (Fortune has asked Facebook for comment, and will include it here when one is received).

The chairman of the Commons media committee, Damian Collins, told the BBC that he had “grave doubts” about the effectiveness of Facebook’s content-moderation systems. He also said that he found it extraordinary that the BBC would be reported to the National Crime Agency when it was just trying to help the company “clean up the network.”

Last year, the BBC did an in-depth report on how pedophiles used Facebook to share images and find others with similar interests by creating private groups devoted to child porn. One group was being administered by a convicted sex offender who was still on the list of registered offenders.

After that report, most of the images and groups the BBC mentioned were not taken down because the social network said they didn’t breach its community standards. Most of the private groups also remained, including one dedicated to photos of schoolgirls with obscene comments. The BBC says that information it provided to police led to one man being sent to prison.

Anne Longfield, the Children’s Commissioner for England, told the BBC that she was disturbed by what she saw in the latest report, and was “very disappointed that one year on we are still seeing images that are very sexualised, totally in my view unacceptable.”