By JEREMY PAGE

BEIJING—Fresh video and still images of China’s supposed stealth fighter prototype have emerged online, fueling speculation among military officials and experts about why the secretive military would suddenly allow the curtain to be lifted on such a sensitive project.

Despite China’s tight Internet controls, Chinese bloggers have posted dozens of images of the J-20 online in the week leading up to a long-delayed visit by Robert Gates, the U.S. defense secretary, who arrives in Beijing Sunday on a mission to repair military ties.

Several Chinese bloggers have also posted what appear to be firsthand accounts of the J-20 conducting high-speed taxi tests—one of the last stages before a test flight—at the Chengdu Aircraft Design Institute in western China.

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