WASHINGTON — A total of 19 people, including 15 identified as Turkish security officials, were indicted Tuesday by a grand jury in the U.S. capital for attacking protesters in May during a U.S. visit by Turkish President Recep Tayyip Erdogan.

The indictments charge the defendants with attacking peaceful demonstrators who had gathered on May 16 outside the home of the Turkish ambassador, awaiting Erdogan’s arrival after he had met with President Donald Trump at the White House. The case threatens to further complicate already tense relations between the two NATO allies.

All 19 defendants are charged with conspiracy to commit a crime of violence, a felony punishable by a statutory maximum of 15 years in prison. Several face additional charges of assault with a deadly weapon.

Sixteen of the defendants had already been charged on June 13; Tuesday’s indictment adds three new defendants, all Turkish security officials. Two of the defendants were arrested in June and face an initial court hearing on Sept. 7. The rest remain at large.

Several are members of Erdogan’s security detail who returned with him to Turkey, so it is unclear if any will face legal repercussions in the U.S.

American officials strongly criticized Turkey’s government and Erdogan’s security forces for the violence. The State Department summoned Turkey’s U.S. ambassador to complain. The Turkish Foreign Ministry then summoned America’s ambassador to protest the treatment of the guards.