A gas leak in an eastern Slovak steel mill killed at least 13 workers and sent 109 to the hospital, officials said Saturday.

Carbon monoxide leaked Friday from the VSZ mill outside Velka Ida, 180 miles east of Bratislava. The gas is a byproduct of plant production.

The leak was caused by an explosion that ruptured a pipe carrying the gas out of the mill. A television report said the pipe was not repaired because plant officials underestimated the danger.

Victims were taken for medical treatment to nearby Kosice. The mayor’s office there said 13 people died, and 244 others required medical treatment. It said 109 people were hospitalized.