(CNN) His voice, his posture and his threats are menacingly familiar.
The black-clad ISIS militant shown in a video demanding a $200 million ransom to spare the lives of two Japanese citizens looks and sounds similar to the man who has appeared in at least five previous hostage videos.
The knife-wielding masked man with a London accent, nicknamed "Jihadi John," has issued threats and overseen the beheadings of American and British captives.
"You now have 72 hours to pressure your government in making a wise decision, by paying the $200 million to save the lives of your citizens," the man in the video that appeared Tuesday says in comments addressed to Japanese citizens. "Otherwise, this knife will become your nightmare."
Q&A: Harsh realities of kidnappings, ransom
The amount of money is the same as that recently pledged by Japanese Prime Minister Shinzo Abe in humanitarian aid to Middle East countries that are affected by ISIS' bloody campaign in Iraq and Syria.
Japan believes the deadline arrives Friday at 12:50 a.m. ET. And Chief Cabinet Minister Yoshihide Suga said Wednesday the country will do its best to communicate with ISIS through a third-party nation.
But mystery and confusion still surround the identity of Jihadi John.
U.S. and British officials have said they believe they know who he is, but they haven't disclosed the information publicly.
That could be because Western intelligence agencies believe they have more to gain from keeping quiet, says Aki Peritz, a former CIA officer.
"They can put pressure on his family, put pressure on his friends," he told CNN. "Maybe they have a line to him. Maybe they know who his cousins are who are going to Syria who can identify him. However, if you publicly tell everybody who he is, his real identity, then maybe he'll go to ground and he'll disappear."
There had been reports that Jihadi John was injured in a bombing attack by the U.S.-led anti-ISIS coalition.
But the video that appeared Tuesday would suggest, if it's really the same person, that he's alive and well enough to make a death threat accompanied by a demand for money.
His apparent reappearance helps ISIS' slick propaganda machine, which is used by the extremist group to raise money and recruit followers.
"It shows them resilient -- that they're able to film this video with these Japanese hostages outside in broad daylight somewhere presumably in Syria," said CNN terrorism analyst Paul Cruickshank.
Five months and roughly 2,000 airstrikes after ISIS' video of the killing of U.S. journalist James Foley, why does one of the West's most prominent tormentors appear to still be at liberty?
U.S. and British military and intelligence officials declined to comment on the matter.
Experts say the problem lies in the extreme difficulty of infiltrating commandos into Syria to reach Jihadi John.
"ISIS operates in what we call denied territory," said Cedric Leighton, a retired U.S. Air Force colonel. "And that means by definition, that it's very hard for our folks to get in there, it's very hard for them to actually lay eyes on target."
A mission by U.S. special operations forces in July failed to find Foley and other hostages in Syria.
"The attempt to rescue Mr. Foley is one of the things that has now made the White House very reluctant to act aggressively," said Tony Shaffer, a former U.S. intelligence operative, who directed special operations in Iraq and Afghanistan.
A senior U.S. administration official, however, said the U.S. government is "actively pursuing justice" for the slayings of the hostages.
"The FBI has an open criminal investigation, and as you have seen, we will hold terrorists like this accountable, no matter how long it takes," the official said on condition of anonymity. "For operational security reasons, we are not in a position to detail all of the steps we are undertaking."
Meanwhile, as ISIS' deadline draws nearer, the Japanese government has to decide how it will respond to the extremists' latest life-or-death ultimatum.