An independent government watchdog with authority of the Federal Reserve has launched an investigation into whether trading by top officials last year satisfied ethics standards and conformed with the law.

The investigation, announced by the Fed on Monday, comes after Jay Powell, chair of the US central bank, announced last month that he would review the institution's ethics guidelines after it was revealed that two officials had actively traded at a time when the institution was aggressively shoring up financial markets.

“As part of our comprehensive review, we began discussions last week with the Office of Inspector General for the Federal Reserve Board (OIG) to initiate an independent review of whether trading activity by certain senior officials was in compliance with both the relevant ethics rules and the law,” the Fed said in the statement.

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Two regional presidents have resigned, Eric Rosengren of the Federal Reserve Bank of Boston and Robert Kaplan of the Dallas Fed, while the trading actions of a third senior Fed employee, vice-chair Richard Clarida, have also come under scrutiny.

“We welcome the [OIG] review and will accept and take appropriate actions based on its findings,” the Fed said.

The revelation of the investigation came only hours after Democratic Senator Elizabeth Warren of Massachusetts requested the top US securities regulator to look into “ethically questionable” transactions by Clarida, Rosengren, and Kaplan and whether they “violated insider trading rules.”