The government believes a video of two Japanese hostages released on the Internet by the Islamic State extremist group may have been doctored, due to such questionable elements as differing shadows on the people in the video.
The government is analysing the video images.
The Islamic State has used high-level technology to edit video clips as part of its promotional efforts, and experts in the field said images of the hostages taken separately were likely combined into a single video to make the footage more intimidating.
"Experts are analysing them [the video images]," Chief Cabinet Secretary Yoshihide Suga said at a press conference Wednesday morning.
It is believed a composite video may have been made using footage of the two hostages in a different place, or in which the hostages were videotaped separately.
In the video clip, two men who are believed to be Haruna Yukawa, 42, and Kenji Goto, 47, are seen kneeling in a desert. A man clad all in black stands between them, wielding a knife and making other intimidating actions.
The shadows of the man on the left side of the screen, believed to be Goto, and the person in the centre extend back and to the right. However, the shadow of the man on the viewer's right, believed to be Yukawa, extends back and to the left.
The shadows of the hostages' heads, which appear on their necks, are also cast in opposite directions.
"It's possible that video images taken at different times were combined," said Tsuyoshi Moriyama, an associate professor at Tokyo Polytechnic University who is an expert on image technology.
"A very high level of knowledge and skill would be necessary to make such a composite video."
Waseda University Prof. Shigeo Mori-shima, an expert on information-communication engineering, focused on the orange clothes the two hostages wore.
The colors were different shades and the two men's clothes did not flutter in the wind at the same time, Morishima said. Therefore, "It's possible that the images [of the two men] were videotaped in different places."
The Islamic State has given itself a sanctified aura, raised the morale of its fighters and recruited extremists across the globe through the use of video clips and other materials on the Internet.
The video of the Japanese hostages was confirmed to have been posted on the Internet on Tuesday, three days after Prime Minister Shinzo Abe announced that Japan will provide $200 million (about ¥23.7 billion) in humanitarian assistance to nearby countries for measures to deal with the Islamic State. Abe is currently on a visit to the Middle East.
Counterterrorism expert Shiro Kawamoto, chief of the Council for Public Policy's second research section, said: "It's possible the militants regarded Prime Minister Abe's announcement of the assistance as a good opportunity and hurriedly put together the composite video. They may have aimed to cause a greater shock than before by having two people appear in the video."
Prof. Kazuo Takahashi of the Open University of Japan, an expert on Middle East studies, mentioned the extremely high ransom demanded in the online video.
"I think they don't intend to negotiate in earnest," Takahashi said.
"The purpose is probably to exhibit their presence and warn Japan, which is taking concerted actions with the United States and European countries," he said.