LOUISA COUNTY, Va. (CBS19 NEWS) -- It has now been about three weeks since someone developed symptoms of illness following a trip to Lake Anna.

That means the Virginia Department of Health is closing its investigation into an outbreak of E.coli that began after the Memorial Day holiday weekend.

According to a release, the most recent testing of water samples collected from Lake Anna did not show the presence of Shiga toxin-producing Escherichia coli, or STEC, O157.

Additional tests for fecal bacteria also did not show levels that would typically rise to public health concerns.

VDH says it was not able to identify the source of the bacteria in the water and laboratory testing did not identify STEC O157 bacteria in water samples from the lake.

The release says the samples used for the STEC O157 testing were collected about four weeks after the holiday weekend, which means they might not reflect the water environment as it was for Memorial Day.

Still, swimming and other activities in any natural body of water pose some health risks because the water has not been disinfected.

VDH will continue to conduct enhanced public health surveillance through July 17.

Additionally, there is a swimming advisory in place for the North Anna Branch of Lake Anna in Louisa County due to an ongoing harmful algae bloom.