Officials have said there is no immediate public health threat because the sewage should be diluted before it reaches parts of the river that are used for the public water supply.
On Saturday, Thomas said officials planned to inspect drainage basins along Antietam Creek for the source of the contamination.
As of Sunday, he said the source has not been found but the solvent had subsided. Whatever the solvent was, it contained chemicals that killed off microbes that are used to treat waste at the plant, causing officials to shut it down, officials said.
No water advisories have been issued to residents of Hagerstown and surrounding areas and people may still use the water in their homes, Thomas said.
McIntire said lab testing most likely will be able to pinpoint what kind of solvent contaminated the plant.
"Good samples were taken, and we'll have an idea of where the stuff may have come from," McIntire said. "We should know what it is."
As a result of the shutdown, millions of gallons of untreated sewage were sent into Antietam Creek, which flows into the Potomac River.
McIntire said state environmental officials on Sunday inspected about 12 miles of Antietam Creek through Funkstown, Devils Backbone and in the Manor Church Road area to see whether the solvent made its way downstream.
He said there has been no discoloration of the water, but that it has started to foam and an odor is present. The Maryland Department of the Environment has ordered signs to be posted along Antietam Creek, telling people to keep away from the water.
If people do come in contact with the water, McIntire said those body parts should be washed thoroughly.
He said there's no indication the water is harming fish or other wildlife and that tests of the water haven't shown anything out of the ordinary.
"We're not really sure of any environmental impact, and we're urging people to stay away," McIntire said.
He said the Hagerstown plant has begun using microbes from the Conococheague plant to help get the contaminated plant fully operational again.
Thomas said the city is working on cleaning equipment in the plant and taking steps to get it running, but it should be a few days before the plant can treat sewage 100 percent.
"It basically has to recover," Thomas said. "It's just like getting the flu."
The Associated Press contributed to this story.