With markets already sinking after a panicky statement from Treasury Secretary Steve Mnuchin on Sunday frantically tried but failed to reassure investors of “ample liquidity,” President Trump threw a grenade into the mix Monday morning with a scathing attack on the Federal Reserve.

“The only problem our economy has is the Fed,” Trump tweeted, less than two days after Mnuchin offered reassurances that the president had no plans to fire Fed Chairman Jerome Powell—a concern that had already rattled investors after Bloomberg reported the president had consulted advisers about the matter.

The Dow Jones Industrial Average briefly plunged more than 500 points after Trump's tweet. By closing, the stock market had dropped 653 points total in the worst Christmas Eve trading ever.

Both Mnuchin and Trump's incoming chief of staff, Mick Mulvaney, had reportedly spent much of the past week trying to steer the president away from ousting Powell. In addition to Mnuchin tweeting late Saturday that the president knew he did not have the "right" to fire Powell, Mulvaney on Sunday told ABC's This Week that Trump “now realizes he does not have the ability” to dismiss Powell.

That apparently did nothing to quell Trump's rage toward the Fed, however.

“They don’t have a feel for the Market, they don’t understand necessary Trade Wars or Strong Dollars or even Democrat Shutdowns over Borders. The Fed is like a powerful golfer who can’t score because he has no touch - he can’t putt!” Trump wrote on Monday.

The president inexplicably decided to take aim at the Fed at a time when markets were already jittery due to a slew of Trump-related factors—the partial government shutdown, the ongoing U.S.-China trade spat, and Trump’s frustrations with the Fed for raising interest rates last week. His comments came after the Dow Jones Industrial Average plunged more than 400 points, a decline some attributed to Mnuchin's cryptic announcement a day earlier.

Mnuchin, who is reportedly vacationing in a luxury resort in Cabo San Lucas, Mexico amid the government shutdown, reassured investors Sunday that he'd spoken to the heads of six major banks: Bank of America, Citi, Goldman Sachs, JP Morgan Chase, Morgan Stanley and Wells Fargo. All of them, he reassured in the least reassuring way possible, had confirmed that they have “ample liquidity.” The treasury secretary also boasted of “strong economic growth in the U.S. economy with robust activity from consumers and business,” apparently in an attempt to prevent a stock selloff.

Buried beneath the rosy assessments in his statement was also the bombshell announcement that he’d be convening the the president’s working group on financial markets—a group more widely known as the “plunge protection group” that was set up in the wake of the October 1987 stock market crash.

The group, which was also convened in 2009 amid the fallout of the 2008 financial crisis, includes officials from the Federal Reserve and the Securities and Exchange Commission.

A source cited by Bloomberg said financial regulators from the group told Mnuchin on Monday that they would monitor the markets during the government shutdown but that they had not seen anything out of the ordinary in the market slump, despite the stock market seeing its worst week since the 2008 crisis.

As of Christmas Eve morning, the Dow Jones Industrial Average, S&P 500, and Nasdaq Composite continued to slide for a fourth session in a row, with the broader S&P 500 index on track for its biggest December drop since the Great Depression.