Mr. Christie criticized politicians who wade into grand jury cases, noting his experience as New Jersey U.S. attorney.

“I’m not going to second guess that work,” Mr. Christie said at a news conference during a visit to Canada. “As someone who ran a prosecuting office for seven years before I became governor, one of the things I learned is that you never know all the things that a grand jury knows, unless you’re in that grand jury and working with them.”