Now George Gascon, who was elected last week to replace Lacey, is reaching out to the group as he prepares for the transition.
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Gascon and BLM organizers met Monday night, along with families of people who died after interactions with law enforcement in Los Angeles County.
"This is a great first step, but we also want to make it very clear that we also plan to hold you accountable," said Melina Abdullah, a co-founder of Black Lives Matter.
Gascon says he will examine cases involving police use of force.
"When we see police violence I think it's very important to understand that yes, it's critical that we address it but we also have to understand that it's part of a larger problem," Gascon said.
There were also some tense exchanges when community members made it clear he'll have to earn their trust.
"Do your job or we will come to your house, we will come to your office," one meeting attendee said.
Gascon said he should be held accountable, adding "the day that you don't think I'm doing my job I'll leave and you can find somebody else."
George Gascon speaks after winning election as Los Angeles County district attorney
While some votes remain to be counted, Lacey conceded defeat earlier this week.
She said the social-justice movement sparked by the death of George Floyd in Minneapolis police custody and other high profile police-involved deaths led to a discussion that's still in progress about racism, policing and criminal justice reform.
"These incidents were painful and exposed an issue that existed in this country for years -- racism,'' Lacey said. "Our nation is going through a reckoning and what happened in my election may one day be listed as a consequence of that. It may be said that one day the results of this election is a result of our season of discontent and a demand to see a tsunami of change.''
Gascon is a former assistant chief with the Los Angeles Police Department who most recently served as the district attorney in San Francisco.
In the campaign, he positioned himself as a progressive reformer and made it clear that prosecution of law-enforcement misconduct would be a priority. If officers commit crimes, he said, "they'll be held accountable just like anybody else.''
"I was not elected to be on the police team or the prosecutors team,'' he said. "I was elected to be on the people's team.''
City News Service contributed to this report.