It was among the most forceful rejections — if far from singular — of Mr. Trump’s remarks on Long Island.

Civil rights activists are closely watching the use of force by the police during the Trump administration. Though most cases occur on the local level, the Justice Department’s civil rights division has the authority to investigate cases of excessive force.

Mr. Sessions has criticized investigations of systemic police abuses and the use of so-called consent decrees, which require specific reforms for police departments and were put in place in more than a dozen jurisdictions by the Obama administration.

Within the first six months of the Trump administration, the Justice Department has closed at least two investigations into possible federal criminal civil rights violations by the police: one related to the 2016 shooting death of a black man in Baton Rouge, La., and the other into the 2014 shooting death of a mentally ill man in Albuquerque. The bar for charging police officers with federal civil rights violations is extremely high, and prosecutions are rare.

The police are a key constituency for the Trump administration, which has highlighted its law-and-order agenda. Some law enforcement officers have cast Mr. Trump’s comments as having been misinterpreted.

“The president’s off-the-cuff comments on policing are sometimes taken all too literally by the media and professional police critics,” said Chuck Canterbury, president of the Fraternal Order of Police. “The president knows, just as every cop out there knows, that our society does not, and should not, tolerate the mistreatment or prejudgment of any individual at any point in the criminal justice process.”

With his email, Mr. Rosenberg may have made himself a target for a president who has forced out a number of officials who have disagreed with him. Even officials who agree with Mr. Trump are vulnerable to his ire; Mr. Sessions has been criticized by the president as “very weak” and “beleaguered” even as the attorney general pushes ahead on the administration’s policies on immigration and violent crime.