Spokesmen for the police and fire rescue service told reporters that apparently Tuesday morning the boy approached cabbie Jonathan Jimenez Gomez, 22, and "without exchanging any words shot him in the forehead" with a pistol.
Jimenez Gomez died at the scene.
The official account was backed up by stunned local residents who saw the murder, and they said that afterwards the boy hid the pistol in his clothing and fled without anyone trying to stop him.
"It was a boy! He was dressed like a boy! He wasn't more than 10 years old," said a shocked police officer, who added that the murder had been recorded on security cameras located on Los Olivos boulevard, which is in the northern part of the capital.
The interior ministry has not yet issued a statement on this killing, the second in less than a week perpetrated by so-called "child assassins" recruited by organized criminal groups because of their legal status as juveniles.
Last weekend, police captured two girls who confessed to murdering a man.
Three weeks ago, a 14-year-old boy was captured with an Uzi submachine gun in his possession while collecting protection payments from businessmen in the southern part of the capital, and two days later a boy of 11 was captured along with two men after committing a murder.