US police have launched an investigation into the killing of another unarmed African-American man by officers outside Baltimore, authorities say.

The mainly black city was rocked by protests and riots over the death of Freddie Gray, a 25-year-old African-American, who died in April from a spinal injury suffered in custody of Baltimore police.

On Thursday, police officers shot and killed Spencer Lee McCain, an unarmed 41-year-old black man, in the Baltimore suburb of Owing Mills. Three officers reportedly fired at least 19 rounds at McCain.

Police said in a statement on Saturday that the Baltimore County police's homicide unit is probing the latest incident.

The police said it will conduct an internal investigation to determine that department rules and regulations were followed during the shooting.

Then, the department will send the findings to the Baltimore County State’s Attorney’s Office for review.

The probe comes at time when US police are under harsh criticism over the use of deadly force, particularly against African-Americans.

The killing of several unarmed black men by white police officers in recent months and decisions by grand juries not to indict the officers triggered large-scale protests across the United States.

The April 19 death of Freddie Gray triggered protests and rioting on the day of his funeral that had continued for weeks. Six police officers are now facing criminal charges in his death.

In a study published last year by Harvard University, the Baltimore metropolitan area ranked near the bottom of major US cities in terms of social and economic mobility.

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