US securities regulators are investigating why Exxon Mobil Corp. hasn’t written down the value of assets during the plunge in oil prices that started in mid-2014, according to published reports.

Both The Wall Street Journal and Bloomberg News, citing unnamed sources, reported the outlines of the investigation by the Securities and Exchange Commission. The Journal said regulators are also looking into Exxon’s calculations for asset values in an atmosphere of tougher climate-change regulations.

Exxon said, ‘‘We are fully complying with the SEC request for information and are confident our financial reporting meets all legal and accounting requirements.’’

Judy Burns, an SEC spokeswoman, declined to comment.

Exxon is also under investigation by Massachusetts Attorney General Maura Healey and her counterpart in New York, Eric Schneiderman, over whether the petroleum giant knew decades ago that fossil fuels contribute to global warming and suppressed that scientific evidence.

The Journal said that the SEC has been receiving documents that Exxon provided to the New York attorney general and that the SEC asked in August for documents from Exxon and its auditor, PricewaterhouseCoopers LLP.

Exxon has long had a policy of not writing down the value of oil and gas reserves when prices fall, arguing it values the reserves conservatively. Last week, a spokesman said the company hasn’t written down the value of its oil and gas fields because Exxon expects them to yield value over the long term.

Other oil companies including Chevron Corp. have taken billions in write-downs in the past two years.

Exxon shares fell 1.5 percent to $82.54 at the close in New York, wiping out more than $5 billion in market value.