The white police officer who shot an unarmed black teenager to death this summer in a St. Louis suburb decided to resign from the force because of threats against fellow officers after a grand jury decided not to indict him, his lawyer said on Sunday.

Darren Wilson's resignation, announced on Saturday, came nearly four months after the officer shot and killed 18-year-old Michael Brown in Ferguson, Missouri, on Aug. 9.

Wilson, who said he was acting in self-defense and that his conscience is clear, had been on administrative leave and in seclusion.

The incident has galvanized critics of the way police and the U.S. criminal justice system treat African-Americans and other minority groups, and led to months of sometimes violent protests in Ferguson and major cities around the country.

A grand jury last Monday decided not to indict Wilson for killing Brown during an August confrontation, sparking widespread demonstrations.

Wilson's attorney Neil Bruntrager said Ferguson's police chief had told Wilson on Saturday that he had information suggesting other members of the department would be harmed if Wilson stayed on the force.

"When Darren was told that, he simply said, 'That's enough,' and it was time to resign," Bruntrager said.

In a letter published by the St. Louis Post-Dispatch, Wilson said he had wanted to wait until after the grand jury's decision before deciding whether to quit.

Even so, his departure was long anticipated because of the potential risks to his own safety and the deep rifts that have surfaced between Ferguson's police and the African-American community since the shooting.

Some critics now want Tom Jackson, Ferguson's police chief, to resign as well, to promote reconciliation in the St. Louis suburb, where most residents are black and the police force are mostly white.

"I think it's impossible for this community to move forward with him still in that role," said St. Louis Alderman Antonio French on ABC's "This Week."

Jackson and Ferguson police could not be reached immediately for comment.

The mayor of Ferguson, Missouri, says Wilson will not receive a severance package as part of his resignation from the police force.

Mayor James Knowles said Sunday that the St. Louis suburb and Wilson have severed their ties and that Wilson won't receive any further pay or benefits following his resignation Saturday.

The Rev. Al Sharpton, a prominent African-American civil rights activist, was likely to speak about the case in a sermon on Sunday at the church in Ferguson where Michael Brown's funeral was held. Brown's parents were expected to attend.

Brown family attorney Benjamin Crump said they would pursue all legal avenues, including a potential wrongful death lawsuit and pushing for a "Michael Brown law" requiring police to wear body cameras to record incidents such as the Ferguson shooting.

"We want police officers who do have a conscience in our community, and not police officers who are cold as ice and see our children as demons and criminals," Crump said on "Face the Nation" on CBS.