The Los Angeles Times

If you checked your Facebook NewsFeed on Monday night and found a dirty Photoshopped picture of Justin Bieber or an image of a beaten and bloodied dog, you are not alone.

Facebook users report that for the last 24 hours the social networking site has been infested with a mix of hardcore pornographic images, doctored pictures of celebrities in sexual situations, photos of extreme violence, and yes, a picture of a beaten dog.

Now, Facebook says it has identified the problem -- but not the culprit.

In a statement, Facebook spokesman Andrew Noyes acknowledged that the site was the target of a coordinated spam attack and explained how it went down.

"During this spam attack, users were tricked into pasting and executing malicious javascript in their browser URL bar causing them to unknowingly share this offensive content," he said. "Our engineers have been working diligently on this self-XSS vulnerability in the browser."

In the meantime, he said that Facebook has built enforcement mechanisms to quickly shut down the malicious pages. He also said the company has also been putting people who were affected by the offensive spam through "educational checkpoints" so they know how to protect themselves in the future.

The images were so graphic that it caused some people to say they were going to quit Facebook.

"If I see another picture of a dead dog on facebook or anything else like that, I will probably delete my account," wrote Big_A22 (Aaron Lichtenwater).

And Fustz (Thomas Morris) tweeted, "Dont really wanna go on facebook anymore, the dead dog made me sad."

The next step for Facebook is figuring out who is behind the attacks. "We are now in the process of investigating to identify those responsible," Noyes said.

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-- Deborah Netburn