Sir Christopher Holland, who served on the high court, concluded in his review of the shooting there was "no lawful justification" for the killing of Azelle Rodney, The Guardian reported. His finding could mean action against the marksman, who is now retired and is identified in documents only by the code E7.
The Guardian said it has also learned that E7 received 5,000 pounds ($7,450) compensation in 2006 after a superior officer jokingly referred to him as a "serial killer."
Former Deputy Commissioner Sue Akers met E7 at a party for an officer who was leaving the force.
"I've always wanted to meet the Met's very own serial killer," she said.
Rodney, 24, was with some other men in a car police were following because they believed they were about to attempt a robbery of Colombian drug dealers. E7 said he fired because he saw Rodney apparently reach down for something and believed he had a gun.
Susan Alexander, Rodney's mother, said she can accept he might have been involved in a crime. She said he should have been arrested.
"The fact that he was strongly suspected in crime does not justify him or anyone else being summarily killed," she said.
Holland said if there was any justification for firing at Rodney it ended after the first shot, which hit him in the right arm.