Chicago Mayor Rahm Emanuel said on Thursday there would be an outside assessment of his administration's lawyers after a federal judge ruled that a city attorney had concealed evidence in a lawsuit over the fatal police shooting of a black motorist.

Emanuel's action followed Monday's resignation of city lawyer Jordan Marsh, who had been singled out by a U.S. District judge for withholding an emergency radio dispatch in a civil trial over the killing of Darius Pinex during a 2011 traffic stop.

Judge Edmond Chang ordered a new trial, reversing a federal jury's decision in favor of Officers Raoul Mosqueda and Gildardo Sierra. It was the second time in less than two months that Emanuel's administration has faced allegations of police cover-ups in shootings.

"The mayor has made it very clear that what happened in this case is unacceptable and we must do everything possible to ensure it never happens again," Emanuel spokesman Adam Collins said in a statement on Thursday.

Collins said the city's top lawyer, corporation counsel Stephen Patton, was finalizing a plan for a third-party review and new training for every lawyer in the Law Department's federal civil rights litigation division.

On Tuesday, Emanuel said he still has faith in Patton, who was Marsh's boss at the time of the trial.

It was unclear who would do the review, what the parameters would be, how much it would cost and if it would limit a civil rights investigation by the U.S. Justice Department to the city's police force. Emanuel said on Tuesday he did not believe the probe should be extended to the city's law department.

Chicago's police force has been at the centre of several cases involving excessive force.

In December, a judge ordered the mayor to release a police dash-cam video that had been withheld by the city during 
Emanuel's re-election campaign. The video showed a Chicago police officer fatally shooting 17-year-old Laquan McDonald, who carried a pocket knife and appeared to be walking away from police.
 
The video and the delay in making it public triggered protests, calls for the mayor's resignation and state legislation that would permit Chicago voters to recall the mayor.