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

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

According to a press release, recent tests of water samples from Lake Anna did not detect Shiga toxin-producing Escherichia coli (STEC O157).

Additional tests for fecal bacteria also did not reveal any values ​​that would normally give rise to health concerns.

According to the VDH, it was not possible to determine the source of the bacteria in the water. Laboratory tests also failed to detect any STEC O157 bacteria in the water samples from the lake.

The press release states that the samples used for the STEC O157 test were collected approximately four weeks after the holiday weekend, meaning they may not reflect the condition of the water on Memorial Day.

However, swimming and other activities in natural waters pose certain health risks because the water has not been disinfected.

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

In addition, a bathing warning is in effect for the North Anna Branch of Lake Anna in Louisa County due to a persistent, harmful algal bloom.