BP PLC is once again in the cross hairs, in a case that could test the limits of U.S. regulators' ability to aggressively go after energy traders.

The Federal Energy Regulatory Commission is accusing BP of manipulating Texas natural-gas markets in 2008. The British energy company denies the allegations and has said FERC—which generally regulates the U.S. power market and large energy-transmission systems—doesn't have jurisdiction over trading that occurs within state lines.

The regulator's enforcement staff is seeking a fine of at least $48 million for alleged violations that brought BP less than $250,000 in profit, according to filings. An in-house FERC judge could rule as soon as this week.

The case represents one facet of a broader push toward greater oversight of physical and financial commodities markets in the wake of the 2008 financial crisis and scandals like the 2001 collapse of Enron Corp. The raft of new regulations brought in to increase transparency and prevent market abuse has turned up the pressure on commodities traders, including BP, one of the world's largest traders of oil and gas. Several Wall Street banks have abandoned or significantly reduced their commodity-trading activities as a result of the increased oversight.

BP and FERC officials declined to comment on the case.

"FERC has been very aggressive in using its anti-manipulation powers," said Craig Pirrong, a University of Houston finance professor. "Other big players in the physical-trading business in particular are watching this carefully."

A finding against BP would be another blow for a company with a long history of run-ins with U.S. regulators. Earlier this month, BP agreed to pay $18.7 billion to settle federal claims from the 2010 Deepwater Horizon oil spill. In 2007, BP settled charges that it had purposely driven up the price of propane, agreeing to pay $303 million and submitting its trading operations to three years of oversight by an independent monitor.

For FERC, the case is the first test of its authority over trading in local physical-gas markets, an area that rarely has been the target of enforcement efforts.

"FERC has taken the path for many years now of being very aggressive on the market-manipulation front and this is really the next step," said Sheila Hollis, a former FERC enforcement director who is now a partner at law firm Duane Morris LLP.

In 2013, FERC brought fines totaling more than $800 million against several banks, including J.P. Morgan Chase & Co., Barclays PLC and Deutsche Bank AG, for allegedly manipulating power markets.

In BP's case, FERC is going after a company that dominates the U.S. natural-gas market.

According to filings, between September and November 2008, BP flooded the Houston Ship Channel delivery hub, located south of the city, with natural gas to push down prices in that location relative to prices at Louisiana's Henry Hub, which sets the U.S. benchmark gas price. BP was willing to take a loss on the physical-gas deliveries in order to profit from a financial-market bet that the gap in prices between those two locations would widen, FERC says in filings.

"The confluence of changed trading patterns...found is not explainable other than by a coordinated manipulative scheme," government lawyers said in a filing. According to the regulator, the alleged scheme cost other market participants as much as $2 million.

In filings, BP has said the trading wasn't improper, adding that FERC's analysis of the trades was flawed.

BP itself brought the matter to regulators' attention when it turned over a recorded telephone conversation between two traders describing the activities.

The recording was a Nov. 5, 2008, conversation between Clayton Luskie, a junior BP trader, and Gradyn Comfort, a senior BP trader. On that day, Mr. Luskie was at a training retreat and happened to describe the trading strategy to another senior company official, who questioned its propriety and suggested he call his supervisor.

From the retreat, Mr. Luskie called Mr. Comfort for advice on how to explain the strategy in a way that didn't "make it sound like we're manipulating" a benchmark index for natural-gas prices, according to a recording submitted as evidence. Mr. Comfort interrupted Mr. Luskie several times during the call, saying there was an economic rationale for the trades. Both men hung up but then resumed the conversation—this time unrecorded—on their mobile phones, according to filings.

BP's trading activities at the time were subject to external oversight, and company executives turned over the tape to the external monitor.

BP says Mr. Luskie misspoke. But trading records obtained by investigators show BP did increase money-losing sales of physical gas at the ship channel while increasing financial bets that price gaps would widen.

Neither Mr. Luskie nor Mr. Comfort could be reached for comment.

In a court filing last month, BP called the regulator's allegations "equal parts hyperbole and overwrought speculative fiction" and said its activities during the period in question were "entirely consistent with its trading during the same time in prior and subsequent years."

If FERC wins, BP could challenge any fine in court, where there is no precedent on whether it has jurisdiction over transactions that don't cross state lines, lawyers say. Market participants increasingly are choosing to litigate rather than settle cases of market abuse.

Write to Christian Berthelsen at christian.berthelsen@wsj.com and Sarah Kent at sarah.kent@wsj.com

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