ROUND ROCK, Texas (KXAN) — A raw sewage spill in Round Rock is responsible for the killing of hundreds of fish in Brushy Creek, according to city officials.

Employees with Round Rock’s Utilities and Environmental Services Department were conducting a regular inspection along Brushy Creek Friday morning and found dead fish at two low water crossings on Red Bud Lane and County Road 123.

An inspection of the area found a manhole near the East Brushy Creek Regional Wastewater Treatment Plant showed signs of having previously overflowed.

City officials believe the spill of raw sewage was the result of a contractor error on Thursday during ongoing expansion work at the treatment plant.

City staff notified the Texas Commission on Environmental Quality and began cleanup efforts. A TCEQ official said approximately 250 fish were killed as a result of the spill.

“At this time, it appears that the contractor working for the City had been working on a line between the plant’s preliminary treatment unit and an aeration basin beginning yesterday morning, and the flow to that line had been shut off during the work,” a city spokesperson told KXAN in a statement. “Due to the length of time that the flow was reduced, wastewater backed up into the collection system’s main interceptor along Brushy Creek and overflowed from the manhole into the creek.”

City staff are reviewing records to determine the duration and volume of the sewage spill.

While the city says this is an isolated incident related to construction, the plant has experienced other issues related to increased flow.

In April, KXAN reported that four leaks at the plant meant the city of Round Rock was dumping more water into Brushy Creek than it is permitted to, meaning it may not be properly treated. KXAN crews also saw untreated sewage in the water in May.

The city says signs have been placed along a portion of the creek downstream from the plant since March, advising residents to stay out of the water.