Pete Buttigieg responded to criticism of his past use of the phrase “all lives matter,” telling reporters Thursday that he stopped saying it when he learned that it was being used to diminish the experience of African Americans.

Following a speech at the Rev. Al Sharpton-founded National Action Network convention in New York, Buttigieg was asked whether it was a mistake to use that phrase given it was often used in opposition to the Black Lives Matter movement.

Buttigieg’s unexpected rise in the Democratic presidential primary means a deeper scrutiny into his past. Buttigieg, 37, is not an official candidate. He is expected to formally declare his run for president on April 14. He announced an exploratory committee in January.

On Wednesday, CNBC reported that Buttigieg, the mayor of South Bend, Ind., said “all lives matter” during his State of the City address in 2015. He said, “There is no contradiction between respecting the risks police officers take every day in order to protect this community, and recognizing the need to overcome the biases implicit in a justice system that treats people from different backgrounds differently, even when they are accused of the same offenses.”

“We need to take both those things seriously, for the simple and profound reason that all lives matter,” he said.

Buttigieg said that when he used that phrase, he did not understand it was being used to counter Black Lives Matter.

“This statement, that seems very anodyne and something that is kind of, nobody could be against, actually wound up being used to devalue what the Black Lives Matter movement was telling us, which is what we needed to hear,” Buttigieg said, adding that he stopped using the phrase after he learned it “was being used to push back on that activism.”

There is a risk that Buttigieg’s use of the phrase in 2015, two years after the Black Lives Matter movement began, could impinge on his inroads with liberals and the African American community.

But during his Thursday speech to the mostly African American crowd, he received immediate applause when he said, “Black lives matter.”

“It should enhance, not diminish, the value of a good police department, when we assert what should go without saying, but in these times must be said clearly and again and again: that black lives matter,” Buttigieg told the convention attendees.

He also endorsed a proposal to form a commission to consider reparations, echoing other Democratic hopefuls who have spoken at the conference. In previous interviews, Buttigieg has said he supports reparations in the form of intentional policies to support the African American community. He has not ruled out some kind of cash reparation but said he didn’t know how that would be implemented fairly.