WASHINGTON — The Syrian opposition on Wednesday began posting about 4,000 photographs of detainees who have died in President Bashar al-Assad’s prisons so that family members can try to identify the victims and potentially serve as plaintiffs in war crimes cases that could be filed in courts in Europe and possibly the United States.

Nearly 27,000 photos of Syrian detainees have been turned over to the F.B.I. for analysis, but the Syrian opposition is now taking the unusual step of publishing those in which the victims’ facial features have not been blurred or otherwise disguised, as they have been in the past because of privacy concerns. The pictures were smuggled out of Syria by a former Syrian police photographer and renowned defector, who uses the pseudonym Caesar.

Secretary of State John Kerry told a United Nations human rights body in Geneva on Monday that the photos Caesar provided show graphic evidence of torture at the hands of the Syrian government.

Many of the faces in the photos are emaciated. Some show signs of beatings. Some of the victims are women, and some are very young. Markings on their foreheads, which were applied by Syrian government officials, indicate the detention center where the prisoners were held and which security agency was responsible for them.