Volcano monitors in the Northern Marianas say ensuring public safety justified them flying to Anatahan island just weeks after its most forceful eruption in history.
Juan Takai Camacho of the Emergency Management Office in Saipan says a helicopter took his team to the island at the weekend.
Mr Camacho says if he hadn't made the trip, officials would still be blind to the dangers posed by the ash plume.
But he says ash and steam meant his team only managed to restore one of the previous four seismic monitoring stations.
"We landed on the east side because of the wind direction, blowing to the west, and all the ash and the steam is moving south west to north west direction, so we were able to land on the east side, where one of the station's located."
Residents of Guam and the rest of the Northern Marianas chain are at risk from toxic ashfalls if the wind changes direction.