Special counsel Robert Mueller’s first and possibly last public statement on the Russia investigation is fueling fresh calls on Capitol Hill to begin impeachment proceedings against President Donald Trump, a step that Democratic leaders have so far resisted.

Surprising Washington with brief remarks on Wednesday, Mueller indicated it’s up to Congress to decide what to do with his findings. The special counsel reiterated that, bound by Justice Department policy, charging a sitting President with a crime was “not an option.”

But he also stressed he could not exonerate Trump. Instead, he cited that same policy to say, “The Constitution requires a process other than the criminal justice system.” With Mueller closing his office and not expected to comment further, it all amounted, for some, to an open invitation for Congress to launch impeachment proceedings.

“We have one remaining path to ensure justice is served,” said Sen. Cory Booker of New Jersey, a Democratic presidential candidate.