WASHINGTON (Reuters) - A grand jury on Tuesday indicted 19 people, including 15 Turkish security officials, in connection with a brawl in Washington between protesters and Turkish President Tayyip Erdogan’s security personnel in May, U.S. prosecutors said.

Eleven people were hurt in what Washington’s police chief described as a “brutal attack” on peaceful protesters outside the Turkish ambassador’s residence during Erdogan’s visit to the United States.

The incident, which took place after Erdogan met with President Donald Trump at the White House, put further stress on already strained U.S.-Turkey relations.

The U.S. Attorney’s Office for the District of Columbia said in a statement charges had already been brought against 16 of the defendants, and Tuesday’s indictment added three to the list.

The three were named as Muhsin Kose, Yusuf Ayar and Harrettin Eren, adding they were among the 15 Turkish security officials indicted. The statement did not specify the others in the list.

“All 19 defendants were indicted on a charge of conspiracy to commit a crime of violence, with a bias crime enhancement,” it said. The charge is punishable by a maximum of 15 years in prison and bias enhancement could lead to longer sentences, the statement said.

Some defendants face additional charges.

Only two of the 19 are in custody, and they are due to appear in court on Sept. 7, the statement said.

The Turkish Embassy has blamed the violence on demonstrators linked to the Kurdistan Workers Party, or PKK, which Turkey and the United States consider a terrorist group.