FERGUSON, Mo. (AP) — The Ferguson police officer who fatally shot Michael Brown has resigned, his attorney said Saturday, nearly four months after the confrontation between the white officer and unarmed black 18-year-old that ignited protests in the St. Louis suburb and across the nation.

Darren Wilson resigned effective immediately, said his attorney, Neil Bruntrager. He declined further immediate comment. Wilson had been on administrative leave since the Aug. 9 shooting, which promoted several days of tense and at times violent protests.

The Brown family attorney, Benjamin Crump, didn't immediately return phone and email messages from The Associated Press.

Wilson told the St. Louis Post-Dispatch he was stepping down out of his "own free will" after the police department told him it had received threats of violence if he remained an employee.

"I'm not willing to let someone else get hurt because of me," said Wilson, who had been with the department for less than three years.

Some witnesses have said Brown had his hands up when Wilson shot him. Wilson told a grand jury that reviewed the case that he feared for his life when Brown hit him and reached for his gun.

The grand jury spent more than three months reviewing evidence before announcing Monday that it wouldn't indict Wilson. The announcement ignited violent protests that resulted in at least a dozen commercial buildings destroyed by fire. Several other large but peaceful protests have since been held in Ferguson and across the country.

The U.S. Justice Department is conducting a civil rights investigation into the shooting and a separate investigation of police department practices.

Wilson's resignation didn't seem to affect protesters outside Ferguson police headquarters Saturday. Rick Campbell said he didn't care, saying: "I've been protesting out here since August." Several other protesters shrugged their shoulders when asked about the resignation.

Ferguson Police Chief Tom Jackson didn't immediately return a message seeking comment.

Wilson spent months in hiding and made no public statements following the shooting. He broke his silence after the grand jury decision, telling ABC News that he couldn't have done anything differently in the encounter with Brown.

Wilson, who recently got married, said he had a clean conscience, because "I know I did my job right." Brown's shooting was the first time he fired his gun on the job, he said. Asked whether the encounter would have unfolded the same way if Brown had been white, Wilson said yes.

Wilson began his career in nearby Jennings before moving to the Ferguson. He had no previous complaints against him and a good career record, according to Jackson, who has called Wilson "an excellent police officer." A few months earlier, Wilson had received a commendation for detaining a suspect in a drug case.