Tommy Yuan was preparing for class at the math-tutoring center he owns in Irving, Texas, one afternoon in June when two men walked in alongside the usual group of students’ parents and then quickly left.

The men returned, identifying themselves as representatives of China’s government and speaking with mainland accents. The older man was plump and wore his hair in a comb-over. The younger one was stocky, had close-cropped hair and looked like he “knew how to fight,” said Mr. Yuan.

“If you want to protect your ex-wife, you’ll give us information,” the visitors said, according to Mr. Yuan, who was born in China and settled in the U.S. in the 1990s. They said they were looking for a man called Ling Wancheng.

Mr. Ling’s brother was a top aide to China’s previous president, Hu Jintao, but was placed under investigation by the Communist Party in December and formally accused in July of bribe-taking, adultery and illegally obtaining state secrets.

For much of 2014, Mr. Ling was living under an alias in a mansion in a gated community in Loomis, Calif., near Sacramento, with Mr. Yuan’s ex-wife, neighbors said. The couple hasn’t been seen there since around October.