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.

By publishing photos of the victims, the opposition is trying to make it possible for relatives to pick them out and, more important for potential legal action, confirm their nationalities. If some of the victims can be shown to have been dual citizens of Britain, Spain, Turkey, the United States or other countries, that would assist the effort to pursue charges for war crimes in courts in those nations, opponents of the Assad government say.

“It is essential that those responsible are brought before a court of law, whether that is The Hague, New York, London or Madrid,” said Toby Cadman, a London-based lawyer who is representing Caesar and his supporters.

The photos are being published on two opposition websites: a Facebook page, StandwithCaesar, maintained by Caesar’s supporters, and a site that focuses on the plight of political prisoners and missing Syrians, www.safmcd.com/martyr/.

“In order to be able to be effective in the pursuit of justice for the victims we must have witnesses and plaintiffs to begin the legal process in national courts where we are planning prosecutions against the Assad regime for war crimes,” said Mouaz Moustafa, who has served as a representative in Washington for Caesar. “It is also important to bring closure for families by helping them identify their missing loved ones.”

Caesar, who is now living in an undisclosed location in Europe, has played a central role in revealing human rights abuses at the hands of the Syrian government. He did not start out as an activist. Caesar was photographing accident scenes for Syria’s military police when the conflict erupted and he was assigned to take pictures of bodies from detention centers, many of which displayed signs of torture. Concluding that he was documenting war crimes, Caesar downloaded copies of the photos and defected.

Obama administration officials believe that the photos are authentic and have praised him for revealing the abuses. In July, Caesar visited Washington, where, wearing a hood to hide his identity, he briefed a congressional panel. He also appeared at the Holocaust Memorial Museum, visited the White House and met with Samantha Power, the American ambassador to the United Nations.

“Anyone who has seen the images will never forget them,” Mr. Kerry said on Monday. “Maimed bodies, people with their eyes gouged out, emaciated prisoners. It defies anybody’s sense of humanity.”

But Caesar’s revelations have not led to concerted international action against the Assad government. Russia’s veto power in the United Nations Security Council has posed an obstacle for referring war crimes allegations against Mr. Assad to the International Criminal Court. In providing the photos to the United States, Caesar and his supporters hoped that the Obama administration would help the legal efforts to hold the Assad government accountable.

In July, Caesar gave 26,948 of his photographs to the F.B.I., and the bureau was to evaluate their authenticity and provide assessments of its findings.

The painstaking work has been difficult, and there is no deadline for completing it, a senior American law enforcement official said in October.

To try to identify the victims, American officials have been using facial recognition software to compare the photos that Caesar provided with visa and passport photos in the State Department’s database and with photos in a separate terrorism database. But only a small number of possible identifications have been made, according to American lawmakers who were briefed on the results last year.

As a result, Caesar’s supporters decided to take matters into their own hands by posting thousands of the photos.

“Regrettably, the F.B.I. has not yet disclosed its findings,” Mr. Cadman said. “That has prompted our team to make a very difficult decision to set up a process by which family members can go through a collection of images with a view to identifying missing loved ones that they believe were arbitrarily arrested, tortured and possibly murdered by the regime.”

If charges are pursued abroad or in the United States, Caesar may also testify, Mr. Moustafa, his representative, said.