A nine-story garment factory in Bangladesh collapsed after a boiler exploded Monday, killing at least 21 people and trapping about 200 others, police and rescuers said.

Rescuers used welding machines to cut holes in the rubble to reach people and pump air to survivors at the sweater-making factory near Savar, an industrial town 20 miles northwest of the capital Dhaka. According to police, at least 300 people were in the factory when it collapsed.