In a letter Thursday to Attorney General Merrick Garland and other officials, the Democrats said a recent Federal Trade Commission investigation into a high-profile merger uncovered evidence of price-fixing by oil executives that led to higher energy costs for American families and businesses.

The FTC said earlier this month that Scott Sheffield, the former CEO of Pioneer Natural Resources, colluded with OPEC and OPEC+ to potentially raise crude oil prices. Sheffield retired from the company in 2016 but returned as CEO in 2019. After retiring again in 2023, he continued to serve on its board.

The FTC cleared Exxon Mobil’s $60 billion deal to buy Pioneer on May 2 but barred Sheffield from joining the new company’s board of directors. Pioneer, which is based in Dallas, said it disagreed with the allegations but would not impede closing of the merger, which was announced in 2023.

In a report, the FTC said collusion by Pioneer and others may have cost the average American household up to $500 per car in increased annual fuel costs, an amount Democrats called “an unwelcome tax that is particularly burdensome for lower-income families.” Meanwhile, Exxon Mobil and other major oil companies collectively earned more than $300 billion in profits over the last two years, “a surge that many market experts believe cannot be explained away by increased production costs from the (coronavirus) pandemic or inflation,” Democrats said.

The letter calls for the Justice Department to launch an industry-wide investigation into possible violations of the Sherman Antitrust Act. It outlined how “Big Oil’s alleged collusion with OPEC is a national security concern that aids countries looking to undermine the U.S.,” including Russia and Iran.

“Corporate malfeasance must be confronted, or it will proliferate,” the letter said. “These alleged offenses do not simply enrich corporations; hardworking Americans end up paying the price through higher costs for gas, fuel and related consumer products. The DOJ must protect consumers, small businesses and the public from petroleum-market collusion.”

The letter by Senate Democrats was the latest in a series of partisan actions targeting the oil industry.

Separately, Democratic Sen. Sheldon Whitehouse of Rhode Island and Democratic Rep. Jamie Raskin of Maryland have formally asked the Justice Department to investigate whether Exxon, Chevron and other oil companies misled the public over decades about the climate effects of burning fossil fuels. Whitehouse and Raskin led a multiyear investigation that uncovered what they described as “damning new documents that exposed the fossil fuel industry’s ongoing efforts to deceive the public and block climate action.”

Republicans, meanwhile, have attacked President Joe Biden’s energy policies, including a freeze on liquefied natural gas exports, restrictions on new oil and gas leasing on a petroleum reserve in Alaska and a decision to charge companies higher rates to drill for oil and natural gas on federal lands.

Sen. John Barrasso, the top Republican on the Senate Energy Committee, said the Democratic president was “doing all he can to make it economically impossible to produce energy on federal lands.”

The letter released Thursday was signed by 23 Democrats, including Schumer, Whitehouse, Senate Commerce Committee Chairwoman Maria Cantwell of Washington state and Senate Judiciary Committee Chairman Dick Durbin of Illinois.