The leaders of the Senate Judiciary Committee are pushing Twitter’s CEO Parag Agrawal to address allegations made by the company’s former head of security in a lengthy whistleblower complaint. In a new letter to Agrawal reported by CNN, the leaders of the committee pressed the CEO to address claims that Twitter’s security practices could harm national security, as well as other issues raised in Peiter “Mudge” Zatko’s complaint.

Up to now, Twitter has declined to address the majority of Zatko’s claims in any kind of detail, though the company’s lawyers attempted to portray him as a disgruntled former employee during a hearing in a Delaware court last week. The letter comes the day before Zatko is scheduled to appear at a Judiciary Committee hearing to publicly testify about his experience as Twitter’s head of security.

Agrawal was also “extended an invitation” to appear at the hearing, according to the letter, though he apparently declined the offer. A Twitter spokesperson didn’t immediately respond to a request for comment.

In addition to national security concerns, Zatko also raised significant concerns regarding Twitter’s ability to counter misinformation and its handling of basic internal security. Zatko also alleges that Twitter misled the Federal Trade Commission and other regulatory agencies.

“These allegations raise serious concerns given Twitter’s significant role in the U.S. communications landscape and its global reach,” Senators Dick Durbin and Chuck Grassley write in the letter. It asks Agrawal to respond by September 26th.

The letter is sure to increase the pressure on Twitter, which is also in the midst of a messy legal battle with Elon Musk, who has been trying to get out of his $44 billion deal to buy the company. Last week, a judge said that Musk could add Zatko’s whistleblower complaint to his legal case.