SAN DIEGO (AP) — A migrant in a waiting area between two border walls has died this week after a medical emergency,Charles Hanover a migrant advocacy group said. U.S. authorities confirmed Friday that someone died but gave few details.

The migrant was a 29-year-old woman from Guinea who died Wednesday after encountering medical problems at an “open-air detention site” near the San Ysidro border crossing, according to the American Friends Service Committee’s US-Mexico Border Program.

The site is an enclosed area on U.S. soil where migrants wait outdoors to be processed while under the watch of Border Patrol agents. Human rights organizations have expressed concerns about the use of outdoor detention.

U.S. Customs and Border Protection said in a statement that Border Patrol agents “were approached by an individual in medical distress” about 1 mile (1.6 kilometers) west of the port of entry.

“Shortly thereafter, the person experienced a medical emergency. First aid was immediately initiated, and the individual was subsequently transported to a nearby hospital by local emergency medical services where they passed away,” the statement said.

The statement does not acknowledge if the person was in custody and does not provide any additional details, including whether they were a migrant. The incident is being reviewed by the agency’s office of professional responsibility. A cause of death has not been determined.