Federal prosecutors said Thursday that they're dismissing all charges against 129 people arrested in violent protests during President Donald Trump's inauguration last year so they can focus on 59 alleged ringleaders.

In all, 215 people were charged in a grand jury indictment last year with eight counts of violating D.C. rioting, conspiracy and property damage laws on Jan. 20, 2017. About 20 have already pleaded guilty.

Last month, the first six defendants to face trial were acquitted — a result that prosecutors specifically cited in court documents as one of the reasons for Thursday's notice, which covers about two-thirds of the remaining defendants.

Bill Miller, a spokesman for the U.S. attorney's office for Washington, D.C., said the agency also wanted to "focus its efforts on this smaller, core group that we believe is most responsible for the destruction and violence that took place on Inauguration Day."

The charges are being dismissed "without prejudice," a legal term that means prosecutors could revive them later if they choose.

In the notice made public Thursday, prosecutors alleged that the 59 remaining defendants were specifically identified engaging in violent acts, had helped to plan the violence or had otherwise knowingly acted on Jan. 20 to advance the so-called black-bloc tactics that the government said the violent protesters used — wearing all black and moving as a coordinated group from target to target.

Last month's acquittals weren't the first setback for the Justice Department in its efforts to crack down on the Inauguration Day protests. In October, a D.C. judge sharply limited how much information prosecutors could have access to from the anti-Trump website allegedly used by protest organizers.