A complaint filed by 16 state attorney generals in 2020 accused Google of illegally cooperating with Facebook, now Meta Platforms, in the market for online advertising, freshly uncensored materials showed.

In a filing updated to account for information contained in internal company documents which has now been unredacted, complainants argue Google and Facebook agreed a deal in 2018 offering the social media company incentives to use the search giant’s advertising auction tool rather than competing products.

The attorney generals claim Facebook won online advertising auctions even when it did not submit the lowest bid, because the fees it paid Google were deeply discounted compared with those paid by other bidders.

Google’s goal was to weaken competing advertising exchanges, the lawyers argue, stating the company biased auctions by “spying and trading based on other bidders’ past behaviour”, a practice made possible by its control of the ad exchange.

The attorney generals also accuse Google of misleading advertisers by falsely stating all bidders were competing equally in its auctions, when it and Facebook had a secret advantage.

Google is also accused of cooperating with Facebook in the market for in-app advertising.