Cody Wilson, whose push to post blueprints for 3-D printed guns online has made him a key figure in the national gun control debate, was charged on Wednesday with sexually assaulting a child in Texas.

But law enforcement officers said they were having trouble finding Mr. Wilson, who missed a flight back to the United States from Taipei, Taiwan, his last known location. During a news conference on Wednesday, Cmdr. Troy Officer of the Austin Police Department said that a warrant had been filed for Mr. Wilson’s arrest and that local detectives were working with national and international partners to find him.

Mr. Wilson, 30, is accused of having sex with a 16-year-old girl at a hotel in Austin on Aug. 15 and paying her $500 in cash, according to an affidavit filed in Travis County. The girl told the police that she had met Mr. Wilson through the website SugarDaddyMeet.com, where he was using the screen name “Sanjuro,” the affidavit says.

Commander Officer said detectives who had interviewed the girl said that “if someone mistakes her age, it would be because they think she’s younger, not older.”