SEATTLE — An accidental release of toxic gas at a manufacturing plant last week that sickened about 120 people probably was caused by workers using the wrong tank for a procedure, Boeing Co. said yesterday.

Boeing said in a statement that a preliminary investigation determined that the accident Friday morning occurred when workers transferred nitric and hydrofluoric acid into an unlined steel tank rather than one with a protective liner.

Boeing officials say 117 workers and neighboring residents were taken to hospitals for treatment of symptoms including shortness of breath, sore throat and dry mouth. At least 12 people were held overnight.

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