The plant that makes 2,000-pound (900 kg) “bunker buster” penetration bombs has stopped production for a second time after workers developed anemia due to TNT exposure, officials said on Wednesday.

Manufacture of the weapons - heavily used in the Iraq war - was stopped on Feb. 8 after resuming a few weeks before following a lengthy production halt, said Mark Hughes, spokesman for the McAlester Army Ammunition Plant.

Blood tests in February found that 17 employees who make the weapons in McAlester had low blood oxygen levels caused by exposure to trinitrotoluene, or TNT, he said.

Production of the powerful bombs, which are designed to destroy underground structures, was stopped in August after 34 workers were found to be anemic.

It restarted Jan. 1 after installation of a new ventilation system to filter TNT from the air, Hughes said.

Initially, practice bombs filled with concrete were made on the reopened line, but when production of bombs began blood tests showed TNT exposure again and the plant was closed, Hughes said.

He said the workers did not exhibit weakness, headaches or other symptoms of full-blown anemia, “but blood lab work indicated clinical levels” of the affliction, Hughes said.

Anemia is a condition in which the number of red blood cells falls below normal and the body gets less oxygen.

Hughes said plant officials are conducting further studies to determine what to do next.

The McAlester plant is the primary bomb maker for the U.S. military and employs 1,400 workers at its 70 square mile site in southeastern Oklahoma.

Hughes could not discuss how many bombs the plant produces or how many bunker buster bombs the U.S. has on hand, but said production of other types of bombs would continue.