Attorney General William P. Barr has assigned an outside prosecutor to scrutinize the criminal case against President Trump’s former national security adviser Michael T. Flynn, according to people familiar with the matter.

The review is highly unusual and could trigger more accusations of political interference by top Justice Department officials into the work of career prosecutors.

Mr. Barr has also installed a handful of outside prosecutors to broadly review the handling of other politically sensitive national-security cases in the U.S. attorney’s office in Washington, the people said.