WASHINGTON — Sen. Richard M. Burr, R-N.C., temporarily stepped down as chairman of the Senate Intelligence Committee on Thursday, a day after FBI agents seized his cellphone as part of an investigation into whether he sold hundreds of thousands of dollars’ worth of stocks using nonpublic information about the coronavirus.

The seizure and an accompanying search for his electronic storage accounts, which were confirmed by an investigator briefed on the case, represented a significant escalation of the inquiry by the Justice Department and the Securities and Exchange Commission. Burr decided to step aside “during the pendency of the investigation” Thursday and informed Sen. Mitch McConnell, R-Ky., the majority leader.

“We agreed that this decision would be in the best interests of the committee and will be effective at the end of the day tomorrow,” McConnell said.

Burr sold the stock in mid-February before the market cratered and while President Donald Trump and some supporters were downplaying the threat of the virus. At the same time, Burr was receiving briefings and involved in conversations suggesting the country faced a burgeoning health crisis that could hurt the economy.

Given the sensitivity surrounding the decision to obtain a search warrant on a sitting senator, the move was approved at the highest levels of the department, according to a senior Justice Department official.

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The warrant to obtain Burr’s phone was served to his lawyer, and investigators took Burr’s phone from him at his home, the official said.

The Justice Department declined to comment. A spokeswoman for Burr declined to comment, and his lawyer did not immediately respond to a request for comment. The senator has denied he did anything wrong, insisting that he based his trading decisions exclusively on publicly reported information that he read in financial media accounts out of Asia.

The Los Angeles Times first reported the existence of the search warrant Wednesday.

Federal investigators have also scrutinized stock trades by other senators around the same time, including Sens. James Inhofe, R-Okla.; Kelly Loeffler, R-Ga., and Dianne Feinstein, D-Calif., according to a person briefed on those cases. All three have said they did nothing wrong.

In April, law enforcement officials asked Feinstein “basic questions” about stock transactions made by her husband during the time period in question, a spokesman for her said Thursday. He said Feinstein complied and provided documents that showed she had no involvement. A spokeswoman for Loeffler said federal authorities had not contacted the senator.

But Burr is the primary target of the investigation, and his case is more advanced, according one of the investigators. Unlike other senators under scrutiny, Burr has not denied that he initiated the sales himself or that they were related to fears about the coronavirus. He has asked the Senate Ethics Committee to investigate.

The intelligence committee chairmanship position is prestigious, and Burr is in the final stages of completing a nearly 1,000-page report on Russia’s 2016 election interference and possible ties to the Trump campaign that is expected to roughly mirror the findings of the special counsel, Robert Mueller.

Burr, whose term ends in 2022, has already said he does not intend to run for reelection.

Democrats in North Carolina renewed their calls for Burr to resign his Senate seat altogether.

“If he has any sense of decency left, Burr will resign immediately, and if they have any regard for the rule of law, Republicans across this state and all over our country will demand the same,” said Wayne Goodwin, chairman of the state Democratic Party.

The investigation stems from Burr’s decision Feb. 12 to sell off 33 stock holdings worth $628,000 to $1.7 million collectively. The sales liquidated a large share of Burr’s portfolio, and they came in the days following a series of senators-only briefings on the spreading coronavirus that Burr received both as chairman of the intelligence committee and a member of the Senate’s Health, Education, Labor and Pensions Committee.

During the intelligence panel briefing, on Feb. 4, CIA officials sketched out an early picture of the geopolitical effect of an outbreak that was still confined to Asia. Other briefings to the health panel, including one the day before Burr’s stock transactions, featured top government health officials who would later steer the country through the pandemic.

Investigators are likely trying to determine the content of those briefings, but it could be difficult. Much of the information may be sensitive or closely resemble public reporting at the time. And experts say Congress’ so-called speech and debate clause could prevent investigators from directly questioning Burr about what he learned in the course of his duties as a senator.

Burr also co-wrote an op-ed article in early February saying that the country was “better prepared than ever” to respond to the coronavirus.