LOS ANGELES — On a 911 call to the Pasadena police Saturday night, the caller said two young black men had put a gun in his face and had stolen his backpack.

When officers responded to the scene, they shot Kendrec McDade, a 19-year-old black man from the nearby city of Azusa, who died of his injuries at a local hospital.

But on Wednesday, the Pasadena police announced that they had arrested the man who made the 911 call, Oscar Carrillo, on suspicion of involuntary manslaughter, because he lied to the police about the suspect being armed.

Lt. Phlunte Riddle said the police now believe that neither Mr. McDade nor his 17-year-old companion was armed. But when officers saw Mr. McDade reach for his waistband, she said, they believed that he was armed and that “their lives were in jeopardy.”