(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." 
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