Sen. Chuck Grassley, the Iowa Republican, led a bipartisan group of senators in sending a letter to the White House late Wednesday seeking a more comprehensive explanation of President Donald Trump’s firing last week of the inspector-general of the intelligence community.

Michael Atkinson, the inspector-general, was instrumental in Trump’s impeachment late last year, having forwarded to Congress a whistleblower complaint concerning Trump’s dealings with Ukraine that Atkinson had found both urgent and credible. Trump, in one of his subsequent remarks on the firing, accused Atkinson of mishandling a “fake report” and characterized the inspector-general as “[n]ot a big Trump fan, that I can tell you.”