The militants appear to have killed one of the two Japanese hostages they have been holding. And they're apparently demanding the release of a convicted terrorist in Jordan to spare the life of the other.
A video file posted online Saturday by a known ISIS supporter shows an image of one hostage, Kenji Goto, holding a photo of what appears to be the corpse of his fellow captive, Haruna Yukawa.
The voice of a person claiming to be Goto speaks over the image, saying in English that Japanese Prime Minister Shinzo Abe is to blame for Yukawa's death.
"You were given a deadline," he says, referring to ISIS' earlier demand that Japan pay $200 million by Friday to save the lives of the two hostages.
The voice then announces a new ultimatum: the release of Sajida al-Rishawi
, an Iraqi woman facing the death penalty in Jordan for her role in a series of bombings in 2005 that killed dozens of people at hotels in the Arab kingdom.
Is the video genuine?
Abe said Sunday that experts were analyzing the video but that it seemed "highly credible." U.S. authorities said they had no reason to doubt its authenticity.
An ISIS-affiliated online radio station, Al-Bayan, reported that the recording was released by the extremist group. It also reported the killing of Yukawa and the prisoner swap demand for Goto in its newscast Sunday. It didn't identify Goto by name.
But Goto's stepfather, Yukio Ishido, raised doubts about whether it was his stepson's voice in the video.
"I get the sense it's not his voice," he told reporters. "I've heard his English a couple times. I felt it was a bit different."
How has Japan responded?
Abe condemned the apparent killing of Yukawa and called for Goto to be released immediately.
"Such act of terrorism is outrageous and impermissible, which causes me nothing but strong indignation," he said.
But the Japanese prime minister declined to comment on how his government would respond to the new demand, which doesn't appear to have a clear deadline.
After ISIS released the first video of the hostages Tuesday, Japan set up a crisis center in Jordan and said it was trying to communicate with the militant group through third parties, such as governments in the region and tribal leaders.
Japanese officials declined to explicitly rule out paying a ransom but said they wouldn't yield to terrorism. They stressed that an aid package Abe had pledged to countries affected by ISIS, cited by the militant group as the reason for the huge ransom demand, was only for humanitarian purposes.
Is the prisoner swap likely to happen?
Observers are skeptical.
"I would be surprised if the Jordanian government or Japan really pushed forward and released this female suicide bomber as ISIS has requested," said CNN global affairs analyst David Rohde, referring to al-Rishawi, whose explosives failed to go off in the hotel attack in which she participated.
CNN law enforcement analyst Tom Fuentes called the swap "a nonstarter," noting that ISIS is already holding a Jordanian pilot whose plane crashed in Syria. Jordan is part of the U.S.-led coalition that's conducting airstrikes against ISIS targets.
"If there is any bargain going to be struck between Jordan and ISIS, it's going to include their pilot," said Fuentes, a former FBI assistant director.
Foria Younis, the chief executive of South Asia Middle East Consultants, highlighted the high profile of al-Rishawi in Jordan.
"This Jordanian prisoner is probably the biggest terrorist prisoner that Jordan has," Younis told CNN.
"To release her, it's just going to require, I think, a lot more than just this one Japanese hostage," she said.
What do we know about the two hostages?
Yukawa, 42, comes across as a lost soul who went to Syria last year in search of purpose after suffering a series of setbacks in his life.
Over the past decade, Yukawa had attempted suicide, lost his wife to cancer, and then lost his home and business.
After changing his first name to its feminine form, he said he believed he was the reincarnation of a Manchu princess who spied on the Japanese in World War II. On a blog, Yukawa once wrote, "I look normal outside, but inside I'm mentally ill."
But his travels to Syria appeared to have been an attempt to rebuild his life. He had dreams of setting up private security company.
It was there that he met Goto, 47, an experienced freelance journalist who frequently reported from war zones. Goto has a wife and two young daughters in Japan.
Despite their contrasting backgrounds, the two apparently became friends in Syria, with Goto helping show Yukawa how to get by in a risky environment.
After Yukawa was reported to have been captured in August, Goto is believed to have decided to go into ISIS-controlled territory, with finding his friend among his goals.
What are their families saying?
Yukawa's father has expressed grief over his son's apparent killing.
"I have been praying such a thing would not happen but, unfortunately, it has finally come to pass and my heart aches," Shoichi Yukawa said in an interview with Japanese media.
"I feel so guilty that Mr. Goto has been detained and threatened death after he had entered there to rescue my son at the risk of his life," he added. "I wish him to be released and return to Japan immediately and continue his activity."
Goto's mother made a plea
to her son's captors to release him on Friday, saying he wasn't an "enemy" of ISIS.
"Words fail to describe how I feel," Junko Ishido said. "Kenji always has been a kind person ever since he was little. He was always saying, 'I want to save the lives of children in war zones.' "
What is the reaction from other countries?
U.S. President Barack Obama spoke with Abe by phone Sunday from New Delhi, the White House said.
Obama offered "condolences for the murder" of Yukawa by ISIS and conveyed "solidarity with the Japanese people," according to a statement.
The U.N. Security Council said in a statement that it strongly condemns the apparent killing of Yukawa, describing it as "a heinous and cowardly act." It demanded the immediate release of Goto.
British Prime Minister David Cameron suggested Japan was right not to bow to the terrorists' demand for ransom, saying that "Britain strongly supports the firm stance Prime Minister Abe and his Government have taken."
The United States and Britain both refuse to pay ransoms to terrorists. But other countries, notably France, have reportedly spent tens of millions of dollars on the release of hostages held by Islamic militants in recent years, despite denying it publicly.
Japan has also paid kidnappers to free its citizens in the past, according to The New York Times. In 1999, Tokyo handed over $3 million for the release of four mining experts held in Kyrgyzstan, the newspaper reported.