Concerns are growing for a trio of missing tourists, including a US citizen, as Mexican authorities question three people in connection with their disappearance.

A vehicle, tents, and a cell phone were found during an ongoing search in the area where the men were last seen, according to Mexican authorities.

American Jack Carter Rhoad and Australian brothers Jake and Callum Robinson were reported missing on April 29, according to the Baja California prosecutor’s office. The three friends are believed to have been on a surfing and camping trip near the municipality of Ensenada, about 60 miles (100 kilometers) south of Tijuana.

“At this moment a team of investigators is in the location where it seems they were seen for the last time, where tents were found along with some evidence that could be related to these three people included in the investigation,” Baja California’s chief public prosecutor Maria Elena Andrade Ramirez told a news conference Thursday.

Three Mexican nationals were being questioned in connection with the case, she said, though it is unclear if they are suspects. Baja California has been plagued by cartel violence in recent years, though it rarely occurs in tourist areas like Ensenada.

In social media posts, the mother of the missing Australians said the three friends have been unreachable since April 27 after they failed to show up at an Airbnb in the nearby coastal resort of Rosarito. She also raised concern about Callum’s diabetes and appealed for anyone with information to come forward.

“Australian friends Jake and Callum Robinson have gone missing around Rosarita/Ensenada region of Baja California Norte Saturday morning,” Debra Robinson posted on Instagram alongside a photo of her two sons.

“They were likely surfing. Callum is diabetic—this is a very dire situation.”

Australian Prime Minister Anthony Albanese expressed concern over the brothers’ disappearance.

“We certainly hope that these brothers are found safely, but there is real concern about the fact that they’ve gone missing,” he told Seven. “Their mother is obviously very distressed about this. And we just hope for a positive outcome.”

Mexican authorities are liaising with US and Australian counterparts, the Baja California state attorney’s office said.