President Donald Trump said Wednesday he has ordered the Department of Justice to investigate major energy companies, accusing them of price gouging by keeping gasoline prices high even as the cost of crude oil has fallen sharply on global markets.

“The big Oil Companies are not dropping their price at the pump commensurate with the sharply lower prices they are paying for Oil,” Trump wrote on social media. “Those prices are dropping like a rock!”

Trump said he told the DOJ to “immediately start looking into this,” adding that he had expected to see petrol prices fall “a lot faster than what I’m seeing.” In his post, Trump argued that customers are being gouged as a result. He did not specify which companies he was referring to.

The announcement comes against the backdrop of the Iran conflict, which has sent oil prices on a volatile trajectory since the U.S. and Israel attacked Iran on February 28. Tehran responded by effectively shutting the critical Strait of Hormuz waterway, severely disrupting oil and gas shipments and sending energy prices sharply higher.

Brent crude, the global oil benchmark, reached almost $120 a barrel in May. The price has since fallen to around $76 a barrel as peace talks progress, though it remains above the roughly $70 mark it stood at before the conflict began.

Meanwhile, the average price of regular gasoline in the U.S. fell to about $3.90 a gallon after topping $4 a gallon in April, according to industry data. While lower than wartime peaks, pump prices remain well above the levels seen before the conflict began, when crude oil was trading around $70 a barrel.

Oil companies have not yet publicly responded to Trump’s allegations. The BBC reported that it contacted the DOJ and the White House for comment. The investigation’s scope and timeline remain unclear.

Trump’s move follows weeks of fluctuating gas and oil prices that have been a recurring focus for the administration. In recent months, Trump has issued mixed messages on the Iran war and oil sanctions, and has at times touted higher oil prices while also expressing concern about the impact on American consumers.