"It was not an operator's error. The (chlorine) cylinders are attached properly," the mayor said.
Stowell said there have been at least two minor chlorine leaks at the plant in the last six years.
"It's possible one of those minor leaks turned into a larger one," the mayor said.
Chlorine is used to disinfect the water. But the gas can be deadly and cause burning in the throat and lungs if inhaled in large enough quantities.
About 10 pounds of the gas escaped from the plant as a result of the leak, Sosnicki and other officials said.
Henry E. Haas, an environmental inspector for the West Virginia Division of Environmental Protection, said it was a "small" quantity of the gas. He said he would "start to worry" if two to three times more chlorine had escaped. He said major leaks usually involve more than 100 pounds of the gas.
"It was a small leak and that was a good thing," Haas said.
He said the gas didn't appear to cause any discoloration of nearby trees or hurt anyone, but it could have if someone had come into contact with the cloud before it dispersed.
The leak was discovered and reported by Sosnicki who turned the plant on Saturday morning.
Sosnicki went to the plant on Bakerton Road a little after 5 a.m. The Harpers Ferry water treatment plant does not operate overnight when no one is there to monitor it, he said.
Sosnicki installed a new chlorine tank onto the system and then turned on the plant.
He said there's always a "whiff" of chlorine around the plant, but Saturday morning it smelled stronger than usual. When he looked outside it "seemed like there was a fog outside by the windows." It was a cloud of chlorine.
The chlorine smell was getting stronger inside the plant, so Sosnicki went outside, drove a couple hundred feet and called 911. It was about 7:30 a.m.
Sosnicki said he could see a cloud of chlorine moving down the hill toward the railroad tracks.
"It wasn't a major leak but I wasn't going to mess with it - not by myself." Sosnicki said.
Scott Biller, second lieutenant with the Friendship Volunteer Fire Company in Harpers Ferry, said when firefighters arrived at the plant there was a small cloud "about 10 feet by 10 feet" next to the plant.
Biller said they closed off Bakerton Road from the intersection with U.S. 340 to an underpass less than a mile from the plant and blocked traffic on nearby Best and Engleswitch roads.
The firefighters, working with self-contained breathing apparatus, shut down the water plant about 8 a.m., he said.
Sosnicki was taken to a nearby hospital as a precaution. He was given a shower and then released, and was back at the water plant within a couple hours.
Biller said the area was deemed safe around 9:30 a.m. and opened to traffic around 10 a.m.
The water treatment plant is in a wooded valley near Elk Run, a small creek that serves as the plant's primary water source. The plant serves about 770 households.
Other emergency crews responding to the incident included the Citizen's Volunteer Fire Co. and the Independent Volunteer Fire Co., both out of Charles Town, the Shepherdstown Volunteer Fire Dept., the Jefferson County Ambulance Authority and the Loudoun County (Va.) Hazardous Materials Team.