How common is 'suicide by cop?'

LAPD Police Chief Charlie Beck told reporters Thursday that 16-year-old Danny Perez, who was shot and killed by police Sunday in South Los Angeles, had a “desire to end his own life.” Police opened fire on Perez after they say he pointed a gun at them, which investigators determined was a fake. Police say that Perez wanted the police to shoot him; in other words, suicide by cop. How often does that happen?