LEXINGTON COUNTY, SC — A transportation company involved in a deadly 2009 chemical leak faces a criminal charge of letting hazardous air pollution leak into the environment at an ammonia plant near Swansea in rural Lexington County.

Werner Transportation Services Inc., of Gainesville, Ga., has been indicted by the federal grand jury in Columbia for allegedly violating the U.S. Clean Air Act, the U.S. Attorney’s Office said late Wednesday afternoon. The company could be fined up to $500,000 for the criminal misdemeanor charge.

In July 2009, Werner Transportation Services negligently released anhydrous ammonia, a hazardous substance, and “negligently placed another person in imminent danger of death and serious bodily injury’’ in violation of federal law, according to the U.S. Attorney’s Office.

Unaware of the leak, motorist Jacqueline Ginyard died after driving through a toxic ammonia cloud in front of the Tanner Industries Inc. plant on U.S. 321 outside Swansea. Ginyard, a health care worker and mother two, was on her way to work when her car was enveloped in the cloud.

The leak occurred after a hose blew out while ammonia was being transferred between the Tanner plant and a Werner tanker truck. Some 7,000 pounds of poisonous ammonia leaked after the wrong type of hose was used to make the transfer, state regulators have said.

At the time, the leak was considered the worst chemical accident in the state since a 2005 train derailment released chlorine and killed nine people near Graniteville.

The spreading ammonia, which can burn people’s lungs, sent at least seven people in the Swansea area to the hospital and caused others to flee from the toxic threat. Leaking ammonia blackened trees and other vegetation for hundreds of yards around the Tanner Industries plant.

Werner officials could not be located Wednesday. A telephone number was disconnected for Werner Transportation Services in Gainesville, Ga. The U.S. Attorney’s Office declined comment late Wednesday.

Tanner Industries, a Pennsylvania-headquartered company that distributes ammonia, also has been under federal investigation.

The S.C. Department of Health and Environmental Control fined Tanner $91,000 in 2010 for a series of emergency preparedness failures connected to the spill. Tanner officials have said they were relying on the trucking company to use the right hose.

The state labor department also has fined Tanner $23,625 for workplace safety violations.