It’s exactly the circumstance Donald Trump tried to avoid. But Trump’s own actions have made an FBI investigation into the president himself a reality.

Firing James Comey, the FBI director, was, by Trump’s explanation, a way to stop a “witch hunt” against his team’s alleged ties to Russia. It led, within weeks, to the appointment of a special prosecutor, Comey’s FBI predecessor, Robert Mueller. And now Mueller is investigating Trump himself for possible obstruction of justice—by firing Comey, who had led the FBI inquiry.

With the crisis engulfing Trump’s young presidency intensifying, senators, Trump aides, former prosecutors, and FBI veterans are sending the White House an urgent warning: Whatever you do, don’t. Fire. Mueller.

News of the obstruction investigation, which was first reported by The Washington Post on Wednesday, comes just days after Trump himself began floating the possibility of firing the new head of the investigation: Robert Mueller, the Justice Department special counsel appointed in the wake of Comey’s firing.

The obstruction investigation has raised the stakes for Mueller’s potential ouster. Firing him now, which would require that Trump personally direct DOJ leadership to do so, would create a political firestorm.

“Firing Robert Mueller right now would be a direct attack on the rule of law by Donald Trump,” Ron Wyden, an Oregon Democrat on the Senate Intelligence Committee, told The Daily Beast. Wyden declined to directly address the Post report.

Trump reportedly floated the possibility of firing Mueller as a way to prod him toward exonerating the president and other Trump associates party to the investigation. The New York Times reported on Tuesday that aides dissuaded him from doing so.

For Ali Soufan, a retired FBI counterterrorism agent, word that Trump is now a target of Mueller’s inquiry explains the trial balloon.

“No wonder President Trump and his surrogates are getting nervous. This explains their sudden attacks on Mueller and the threats to fire him,” Soufan told The Daily Beast.

White House officials are still insisting to the president that he should leave Mueller in his post. “We are all advising him not to [get rid of] Mueller. That has not changed,” one Trump aide told The Daily Beast. “It would be an absolute nuclear explosion if he did.”

Firing Mueller would also put the president in greater legal jeopardy than he already may be in, said former United States attorney Barbara McQuade.

“If Trump were to fire Mueller and it could be shown that his purpose was to impede the investigation, it could be additional evidence of obstruction of justice,” McQuade, who was appointed by President Obama, told The Daily Beast.