Agents with the Federal Bureau of Investigation (FBI) seized Sen. Richard Burr’s (R-NC) cellphone on Wednesday as part of an ongoing investigation into the lawmaker’s decision to unload upwards of $1.72 million in stock ahead of the Coronavirus outbreak.

Burr, who serves as chairman of the Senate intelligence committee, was reportedly served a warrant for his cellphone at his Washington, D.C., residence on Wednesday, according to the Los Angeles Times. Law enforcement officials purportedly seized the phone in order to examine any communication the senator may have had with his stockbroker over the past few months.

“Such a warrant being served on a sitting U.S. senator would require approval from the highest ranks of the Justice Department and is a step that would not be taken lightly,” the Times noted.

The seizure is the first significant development since the Department of Justice and the Securities and Exchange Commission announced last month they were investigating the Republican lawmaker for possible insider trading.

Burr drew the attention of lawmakers after it was revealed he sold off thousands of dollars worth of stock on February 13—less than a week before the stock market sharply dropped because of the coronavirus pandemic. Most of the shares were in companies  like Wyndham Hotels and Resorts and Hilton that took an especially hard hit as coronavirus travel restrictions went into place. Burr’s timely decision to sell netted him between $628,000 and $1.72 million.

More troubling is that Burr’s decision to sell came as his committee was receiving daily briefings on the threat posed by the virus. As such, many speculate the senator may have acted on insider information to protect his assets. If true, Burr could be found in violation of the STOCK Act, which prohibits the use of non-public information for private profit by lawmakers.

The FBI’s seizure of Burr’s phone comes only days after it was reported that his brother-in-law, a political appointee of President Donald Trump, dumped upwards of $280,000 in stock on the same day as the senator.