BAKERSFIELD, Calif. (AP) — A 74-year-old South Korean man has died of apparent suicide at a U.S. immigration detention center in California after advocates said they unsuccessfully sought his release.

The advocates had said the man was at high-risk of being infected with the coronavirus. U.S. Immigration and Customs Enforcement says Choung Woong Ahn was found unresponsive in his cell Sunday at the Mesa Verde ICE Processing Center in Bakersfield.

The preliminary cause of death is self-strangulation but the case remains under investigation.

Advocates say Ahn had diabetes, hypertension and heart-related issues. ICE said Ahn became permanent U.S. resident in 1988 and was convicted in 2013 of attempted murder.