Alan Dershowitz said Thursday that bringing in Russia special counsel Robert Mueller was "an insult to the existing civil servants in the Justice Department, to suggest that they can't do the job well."
"You really should have to show an extraordinary need to go outside the usual channels of the Justice Department to justify bringing somebody in and giving that person a special task," the Harvard Law School professor emeritus told Anderson Cooper on CNN.
"The Southern District of New York has handled many, many more complex cases than that."
Dershowitz said Thursday that the leaked draft of Justice Department Inspector General Michael Horowitz's report proved no special counsel was needed to investigate Moscow's meddling in the 2016 president election.
President Donald Trump later tweeted Dershowitz's conclusion was "so true."
The retired professor told Cooper that various U.S. attorney's offices around the country have handled far more extensive investigations that Russia.
"There are so many complex cases, international cases," Dershowitz said.
"It really is an insult to the existing civil servants in the Justice Department, to suggest that they can't do the job well.
"Especially when you have an IG looking over them," he added. "The president couldn't complain."
But Dershowitz then said that Trump "would, to be sure" attack any similar probe. "He'd complained about anything.
"But you have less of a basis for complaining if it was a routine investigation, done by the Southern District and other districts."