Attorney General Jeff Sessions should explain whether the firing of U.S. Attorney Preet Bharara had anything to do with Bharara’s reported probe into stock trades by Health and Human Services Secretary Tom Price, three Democratic U.S. senators said Tuesday.

Price came under scrutiny during his confirmation hearings for investments he made while serving in Congress — including hundreds of thousands of dollars worth of shares in health-related companies, even as he voted on and sponsored legislation affecting the industry.

In their letter to Sessions, Sens. Elizabeth Warren, Jeff Merkley and Richard Blumenthal referred to a ProPublica story, which cited a person familiar with the office as saying that Bharara was overseeing an investigation of Price’s actions. They asked whether Sessions, President Trump or other officials in the Justice Department or White House were aware of such a probe before they removed Bharara, the top federal prosecutor in Manhattan.

Bharara was one of 46 U.S. attorneys asked to resign after Trump took office. It is not unusual for new presidents to replace federal prosecutors with their own appointees, but Bharara’s firing came as a surprise because the president had met with him at Trump Tower soon after the election. After that meeting, Bharara told reporters Trump asked if he would be prepared to remain in his post, and said that he had agreed to stay on.

After his dismissal, Bharara fanned suspicions that the move was politically motivated via his personal Twitter account.

“Now I know what the Moreland Commission must have felt like,” he wrote, referring to a commission that was launched by New York Gov. Andrew Cuomo only to be shut down by the governor after its work grew close to his office.

When Price was asked about ProPublica’s report earlier this month on ABC News’ “This Week with George Stephanopoulos,” he said he and his lawyers haven’t received any indication of a federal investigation.

In their letter, the senators said they “have questions about whether such an investigation had any impact on President Trump’s decision to fire Mr. Bharara.”

They included a list of questions about who made the decision to fire Bharara, who knew about the Price probe, and whether Bharara was overseeing any other investigations relating to Trump.

“Was Mr. Bharara involved in or conducting an investigation of Secretary Price at the time he was fired?” they asked. “Was President Trump aware of the investigation at the time Mr. Bharara was fired?”

The senators asked Sessions to respond no later than April 3.

The Department of Justice and the White House did not immediately respond to requests for comment.

Price has said that his trades were lawful and transparent. Democrats accused him of potentially using his office to enrich himself. One lawmaker called for an investigation by the Securities and Exchange Commission, citing concerns Price could have violated the STOCK Act, a 2012 law signed by President Obama that clarified that members of Congress cannot use nonpublic information for profit and requires them to promptly disclose their trades.

Price has come under particularly intense scrutiny for shares he bought of Innate Immunotherapeutics — a tiny Australian company working on an experimental multiple sclerosis drug. As a congressman heavily involved in health policy, he was just one of a handful of American investors allowed to buy discounted stock in the company.