The pilot of a medical helicopter twice radioed for help in foggy weather before crashing Sunday, killing four of the five people on board in the latest of a growing number of air ambulance accidents, authorities said.

The medical helicopter was carrying victims of a traffic accident when it went down in a suburban Washington park. It was the deadliest medevac helicopter accident in Maryland since the State Police began flying those missions nearly 40 years ago, and the eighth fatal medical helicopter crash in the last 12 months nationwide. About 30 people have died in such crashes during that period, National Transportation Safety Board member Debbie Hersman said.