WASHINGTON — Law enforcement officials plan to announce charges Thursday against a dozen members of the Turkish president’s security detail for their involvement in a brutal attack on protesters outside the Turkish ambassador’s residence here last month, two American officials said on Wednesday.

The authorities have already charged several others, including two Americans and two Canadians, with taking part in the violent skirmish.

The Washington police have been investigating the May 16 attack along with the State Department and the Secret Service. The police planned to announce the charges at a news conference on Thursday morning, according to the two officials, who spoke on the condition of anonymity to discuss the charges before they were made public.