Murray said the MDE is asking about the county's use of the degreaser Pinetech E because county officials informed the state that the degreaser was used around the time of the city's problems.
Pinetech E, which is made from the bark of pine trees, is biodegradable and made for use in sewer systems, he said.
During the past eight years, the county has used Pinetech E about three or four times a year and never had any problems, he said. The product is used in the county-maintained sewer systems connected to the city's and county's treatment plants, he said.
The county used the degreaser in the sewer system north of Hagerstown, which is connected to the city plant, during the week the problems occurred at the city plant, Murray said.
In addition to the product's safe track record, Murray said Pinetech E does not contain any of the chemicals believed to have caused the problems at the city plant.
In a letter received by city and county officials during the past week, the MDE asked the county for "All reports, records and documents regarding the application of Pinetech E degreasing agent or other degreasing agents applied to any portion of the Washington County sewage collection system ... during the period Feb. 5 through Feb. 9."
The MDE also asked for information about Pinetech E, and "All available reports, records and documents for the past three years, pertaining to the maintenance, cleaning and degreasing" of the sewage system.
MDE spokesman John Verrico said the county government was the only entity to receive such a letter from the MDE.
"This is all still an information-gathering point of this investigation. There's not any finger pointing yet," he said.
The agency still does not know who or what caused the partial shutdown of the city plant, he said.
The Hagerstown sewage treatment plant off Frederick Street was partially shut down Feb. 9 after high concentrations of chemicals common to industrial cleaners and other industrial products were dumped into the sewer system.
The screening and settling steps of the treatment process, which remove sludge and other solid material, continued operating during the partial shutdown. But the disinfection and bacteria treatments were stopped, because the toxic chemicals killed the sewage-eating bacteria used to break down complex bacteria and chemicals at the plant.
More than 15 million gallons of largely untreated sewage flowed from the plant and into Antietam Creek during the partial shutdown.
A chlorine disinfection process was put in place Feb. 12, which dramatically lessened the bacteria levels flowing into the creek.
Plant operations have been back to normal since about March 1.
In another letter received by city and county officials, the MDE asked city officials about procedures to monitor for, detect and minimize the impact of toxic pollutants that could come into the plant in the future.
"They're asking the city what can be done to prevent this from happening again and how we could respond (better)," Mayor William M. Breichner said.
Breichner said city sewer plant staff could switch to a chlorination disinfection process sooner if such an incident happened again. But he said it would be "very difficult" to completely prevent such an incident in the future.
"Even if we do everything required, we'll still have some susceptibility" to a future incident, city Water Pollution Control Department Manager Rick Thomas said.