Facebook knowingly inflated video ad metrics and lied to advertisers about it, according to a group of those advertisers.

As The Wall Street Journal reports, the advertisers, who sued Facebook in 2016 for overstating its metrics, now claim Mark Zuckerberg and Co. discovered that they'd been over-reporting the average amount of time users spent watching paid video ads in January 2015 but withheld that information for more than a year.

"Internal records recently produced in this litigation suggest…that Facebook's action rises to the level of fraud and may warrant punitive damages," the advertisers said in a Tuesday filing.

The controversy dates back to September 2016, when the Journal reported that Facebook had been overestimating average video ad viewing times for two years. Facebook claimed it calculated the "average duration of video viewed" by dividing the total time users' spent watching a video by the total number people who watched it. In reality, the company only included those who had viewed a video for three seconds or more when calculating this metric, leaving out everyone who clicked away before three seconds.

At the time, Facebook claimed it only discovered the problem "about a month" earlier and had fixed it right away. The advertisers, however, say otherwise.

"Facebook did not discover its mistake one month before its public announcement," they wrote. "Facebook engineers knew for over a year."

The plaintiffs went on to say that "multiple advertisers had reported aberrant results caused by the miscalculation," but the social network "did nothing" to stop the spread of its incorrect metrics.

Facebook did not immediately respond to PCMag's request for comment but denied the allegations in a statement to the Journal.

"Suggestions that we in any way tried to hide this issue from our partners are false," a Facebook spokeswoman reportedly said. "We told our customers about the error when we discovered it—and updated our help centre to explain the issue."

Meanwhile, the filing further claims that the problem was more severe than previously reported. When the issue came to light, Facebook said it overestimated average time spent watching video ads by as much as 60 to 80 percent. In their Tuesday filing, the advertisers say it's more like "150 to 900 percent."

As a number of journalists have pointed out on Twitter today, video metrics from companies like Facebook prompted a number of media outlets to lay off print reporters and "pivot to video" in recent years, a strategy that has yet to pay off.