Montgomery, Ala. ? A seventh police officer resigned Friday amid an investigation into allegations of police brutality.

Four other officers have been placed on paid leave since the probe began Monday. The officers have been accused of abusing their positions of authority, mistreating citizens and false reporting of incidents.

All 11 patrolled in different parts of the city on the graveyard shift.

Larry Armstead, an assistant to the mayor, said all the victims were black. But he said he couldn’t comment on whether the investigation sought to determine if the alleged crimes were racially motivated. Eight of the 11 officers are white, as are all seven of the officers who resigned.

Officer Michael Clark, 32, was charged Monday with criminal use of Mace after scuffling with a 17-year-old prisoner he was transporting. Police said an argument prompted Clark to stop the car and the shoving match ensued. Clark was jailed and released on $1,100 bond before he resigned.

Armstead said some of the charges include conduct worse than the Mace-spraying incident, and go back several months. He declined to give further details.

Montgomery Police didn’t return calls Friday. City attorney Howard Mandell was out of his office and couldn’t be reached for comment.

The probe began after an officer reported to his supervisor that his colleagues had been mistreating people, Armstead said. The department has asked the FBI and the district attorney’s office to begin a criminal investigation.