WESTBROOK — S.D. Warren will not be cited for an accident that killed a worker this spring, federal workplace safety regulators said Wednesday.

The Occupational Safety and Health Administration investigated the March death of 46-year-old Fred McKeeman at the Westbrook mill.

He was killed when a large steel grate gave way and the forklift he was operating crashed 23 feet into the basement.

OSHA said that the accident was caused by a structural problem that mill operators could not have known about. The regulators said a bent beam at the mill could have been that way since the building was built in 1981.

“The condition appeared quite innocuous when first observed,” said C. William Freeman III, OSHA’s area director for Maine. “It only became apparent on close examination of the beam just how hazardous the condition was.”