The weather alert system and runway lights weren’t working at a foggy San Diego airport where a private jet was planning to land before it crashed into a neighborhood, but the cause of the crash remains unclear,

The San Diego plane crash that killed at least four people and injured several others on Thursday involved a Cessna 550 private jet. The crash took place around 3:45 a.m. in the Murphy Canyon military housing neighborhood.

Dense fog had rolled into the area around the time the plane was approaching the airport. Visibility was at half a mile at 3:55 a.m. and had dropped to a quarter of a mile just after 4 a.m. in the area around Marine Corps Air Station Miramar, which is just a few miles north of the crash site, according to the National Weather Service.