WASHINGTON — The FBI has seized the cellphone of Sen. Richard Burr, R-N.C., pursuant to a search warrant as part of an investigation of possible insider trading, a senior law enforcement official confirmed to NBC News.

The Los Angeles Times first reported Wednesday night that federal agents obtained the cellphone belonging to Burr, indicating a major escalation of the Justice Department investigation into the stock trades.

The newspaper reported that Burr turned over his phone to agents after they served a search warrant at his Washington-area residence, citing a law enforcement official it did not identify.

A spokesman for Burr declined to comment Wednesday. A spokesperson for the Justice Department also declined to comment on The Times story.

Burr, who chairs the Senate Intelligence Committee, faced calls to resign in March after reports that he privately warned well-connected donors of the dire impacts of the coronavirus pandemic in February while selling off up to $1.6 million of his own stocks.

Burr said in March that he relied on public news reports, not inside information from his role on the Senate committee.

He said he asked the chairman of the Senate Ethics Committee “to open a complete review of the matter with full transparency.”

Alice Fisher, a lawyer who is advising Burr, said in a statement in March:

“The law is clear that any American — including a senator — may participate in the stock market based on public information, as Senator Burr did. When this issue arose, Senator Burr immediately asked the Senate Ethics Committee to conduct a complete review, and he will cooperate with that review as well as any other appropriate inquiry. Senator Burr welcomes a thorough review of the facts in this matter, which will establish that his actions were appropriate.”

Burr has previously announced he would not seek re-election. He was last elected in 2016, and the next election for his seat is in 2022.

The furor began after ProPublica, a non-profit news organization, reported that Burr unloaded the stock around mid-February, about a week before the market started to plunge because of coronavirus concerns.

That included selling off up to $150,000 worth of shares of Wyndham Hotels and Resorts and $100,000 of shares of another hotel chain, Extended Stay America.

ProPublica discovered, and NBC News has confirmed, the stock sell-offs in Burr’s publicly available financial disclosure reports. The exact figures are unclear because the reports offer ranges of transactions.

Disclosure records also show that three other senators sold major holdings around the same time, including Dianne Feinstein, D-Calif., Kelly Loeffler, R-Ga., and James Inhofe, R-Okla., according to The New York Times.

Loeffler has defended the sales and said, “investment decisions are made by multiple third-party advisors without my or my husband’s knowledge or involvement.” Her husband is chairman of the New York Stock Exchange.

A Feinstein spokesman in March said that the senator did not sell any stock and that “the transactions you’re referencing were made by her spouse.”

The spokesman said all of Feinstein’s assets are in a blind trust, and it’s been that way since she came to the Senate.

Burr was first elected to Congress in 1994 and served five terms in the House of Representatives before running for Senate; he was elected to that body in 2004 and is in his third term.

Burr, 64, told reporters before his win in 2016 that if he were successful in getting a third term, he would serve out his six-year term and then retire.

Burr, while a Republican, has occasionally taken actions that have angered some of the more fervent supporters of President Donald Trump.

A bipartisan investigation by the Senate Intelligence Committee released recently validated the January 2017 U.S. intelligence assessment describing Russia interference in the 2016 presidential election and described that assessment as describing it as accurate, thorough, and untainted by political bias.

“The Committee found no reason to dispute the Intelligence Community’s conclusions,” Burr said in a statement at the time of the report’s release in April. That report by the Republican-run committee examines how the assessment was put together.

Wednesday night U.S. Rep. Matt Gaetz, R-Fla., who is a vocal defender of Trump, seemed to tweet about the stock controversy, using the term “stock selloff collusion.”

Gaetz also retweeted a comment from right-wing media figure Mike Cernovich who wrote that Burr “tormented prominent Trump supporters,” including Trump’s son, Donald Trump Jr.

Trump Jr. was subpoenaed by Burr’s committee in 2019 to answer questions about his past contention that he had only limited knowledge of a project to build a Trump Tower in Moscow.

Pete Williams reported from Washington, and Phil Helsel from Los Angeles.

Julie Tsirkin, Kasie Hunt and Michael Kosnar contributed.