One cause investigators are looking into is whether a homeowner doing plumbing work without a permit might have prompted the bacteria to appear in a water sample taken at a house on Coronado Way in central Cary.
The homeowner, authorities said, is cooperating with investigators and is unlikely to be charged if the unauthorized plumbing work is determined to be the cause, but he could be fined anywhere from $50 to $1,000 for not obtaining a permit.
"There's a reason why plumbers need to be licensed," said Cary Public Works Director Mike Bajorek. "You are working on a sanitary system in close contact with potable water and any mixture of that could cause trouble."
E. coli infection can cause diarrhea, cramps, nausea, headaches and other symptoms.
Although the bacterium showed up at only one residence, town officials invoked a boil-water order for the entire town on Friday afternoon as a precaution. About 550 businesses were forced to close.
Area hospitals reported no cases of E. coli related to the scare, officials said.
Early Sunday evening, the town lifted the order for all but 12 residences, which are in the Coronado Village subdivision. It was not immediately clear on Monday evening when the order would be lifted for residents there.
Although some people have wondered whether the town overreacted to the E.coli threat, state and Wake County authorities said on Monday that the town followed the proper guidelines followed and put appropriate precautions in place.
"It was a regulated thing to do, as well as the right thing to do," said Andy Pierce, director of Wake County Environmental Services.
Wake County Environmental Services did, however, express concern that it was notified about an hour after the media and had to scramble to notify businesses about closing.
Town leaders said on Monday that Cary is already learning from its mistakes in how it handled Friday's situation.