On Thursday, almost all traders in the West will be closely watching the latest U.S. consumer price index (CPI) data from the Labor Department, due at 08:30 a.m. ET (13:30 GMT), Bloomberg writes.

But even a month earlier – on Dec. 13 – before the monthly CPI, one of the U.S. Federal Reserve’s preferred measures of inflation, was released, something unusual happened: trading volume in 10-year Treasury bond futures soared to 3 times the level seen a minute before all 24 CPI reports were released.

“The trading volume was pretty extraordinary,” said J. Christopher Giancarlo, senior counsel at the law firm Willkie Farr & Gallagher.

Meanwhile, the Labor Department dismissed rumors of a possible data leak after a preliminary investigation by the Commodity Futures Trading Commission (CFTC) and the Securities and Exchange Commission (SEC).

Both agencies said they monitor Treasury futures market movements on a daily basis, especially ahead of the next release of the Consumer Price Index.

However, no one has been able to explain what happened then, but speculation remains that the surge in trading was caused by a hack or leak.

According to Graham Harper, head of public policy and markets at Chicago-based trading firm DRW, “The current mechanism for publishing economic data raises doubts about the integrity of the process.”

Research has shown that the report has triggered stock market swings so sharp that they have not been seen in a decade, causing a negative response to economic indicators like the CPI.

The set of numbers released today is important because it will be a key factor in determining whether Fed officials raise the interest rate by half a percent for the second straight meeting or lower it to a quarter-point. Economists’ average CPI forecasts call for a 0.1% decline in inflation and a 0.3% rise in the so-called core CPI, which excludes food and energy prices.

Now that the CPI will be released, many traders will be more vigilant, even though the official sector may assure that everything is fine and nothing has actually happened. Following the payrolls data, all the responsibility for the Fed rate hike now lies with the inflation data.