Facebook has published internal emails offering a glimpse into when employees became aware of Cambridge Analytica's behavior.

The message thread was first reported by NBC's Dylan Byers, although Facebook published the same document less than an hour later.

Facebook says the emails show Mark Zuckerberg's testimony to Congress last year was correct when he said the company was not aware data was sold to Cambridge Analytica until it was reported by The Guardian in December 2015.

The emails are also a stark reminder that Facebook employees understood the gravity of the Cambridge Analytica issue in 2015, but the firm failed to properly act until the story exploded in March 2018.

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Facebook has published internal emails that shed new light on exactly how Cambridge Analytica came onto its employees' radar, more than two years before the data scandal sparked the worst crisis in the firm's history.

NBC's Dylan Byers first obtained a copy of the internal Facebook message thread where employees discussed how political partners accessed user data, but it was made public by the company less than an hour later.

It shows that employees talked about an investigation into possible data scraping by political partners in September 2015 after being made aware of the issue by an unnamed individual not employed by Facebook.

This person, whose name has been redacted, said in a message dated September 22 that Cambridge Analytica is "the largest and most aggressive [company] on the conservative side" of the industry "matching social data to voter files."

The email described Cambridge Analytica as "a sketchy (to say the least) data modeling company that has penetrated our market deeply."

Read more: We talked to the professor who fought Cambridge Analytica to get his data back in Netflix's 'The Great Hack' about why privacy rights in the US are lagging behind the rest of the world

A message thread ensued on September 29, in which employees debated whether the data scraping constituted a violation of Facebook's rules. There was no clear conclusion.

The emails show that it was not until December 2015 that Facebook became aware from a report in The Guardian that user data scraped by Aleksandr Kogan — through his This Is Your Digital Life quiz — was sold to Cambridge Analytica.

One employee said Facebook had not heard of Global Science Research, the company set up by Kogan that sold Facebook user data to Cambridge Analytica, prior to The Guardian article being published.

Another employee wrote: "Can you expedite the review of Cambridge Analytica or let us know what the next steps are? Unfortunately, this firm is now a PR issue as this story is on the front page of the Guardian website." A third member of staff added that "we need to sort this out ASAP."

Alexander Nix, former CEO of the now-defunct Cambridge Analytica. Pedro Nunes/Reuters

Facebook says this timeline is important because it shows that CEO Mark Zuckerberg's testimony to Congress last year was correct.

"Facebook was not aware that Kogan sold data to Cambridge Analytica until December 2015. That is a fact that we have testified to under oath, that we have described to our core regulators, and that we stand by today," Paul Grewal, Facebook's deputy general counsel, said in a blog on Friday.

Zuckerberg's evidence was thrown into doubt after the Securities and Exchange Commission last month filed a complaint against Facebook specifically alleging that employees had "requested an investigation into possible 'scraping'" in September 2015.

The emails may help clarify this issue for Facebook, but they are also a stark reminder that employees understood the gravity of Cambridge Analytica's behavior in 2015, but the firm failed to properly act until the story exploded in March 2018, when whistleblower Christopher Wylie lifted the lid on the scandal. Wylie's revelations about the information of 50 million users being exploited by Cambridge Analytica sparked the worst crisis in Facebook's history.

Grewal acknowledged that Facebook got it wrong in his blog on Friday."Cambridge Analytica was a clear lapse for us, which we have worked hard to address. We've learned many lessons that will help us become a stronger company going forward," he said.

You can read the Facebook emails here: