BEIJING — Gao Zhisheng, a prominent human rights lawyer whose 13-month disappearance at the hands of Chinese security agents stirred an international outcry until he resurfaced in March, has again vanished, his friends said Friday.

Associates said Mr. Gao failed to return to a Beijing apartment on April 20 after spending more than a week in Urumqi, the capital of the Xinjiang region in western China, where he had been visiting his father-in-law. Mr. Gao telephoned his father-in-law as his plane left Urumqi, saying he would call upon his arrival in Beijing, they said.

That appeared to be his last contact with the outside world. Li Heping, another Beijing human rights lawyer and a close friend, said he had visited Mr. Gao’s apartment repeatedly, but had not found him. “No one had been there for a while,” said Mr. Li, who last went to the apartment on Thursday. “I have no idea who to call, or who has taken him.”

Others said they were sure that the government had again removed him from public view and that the authorities’ earlier decision to allow him to resurface briefly had been a ploy to try to demonstrate to the outside world that he had not been mistreated.