A hazmat incident at a Riverside, California, pool injured up to 25 people and sent three children to the hospital by ambulance.

The incident occurred on July 17 at the Ysmael Villegas Community Center Pool when the water bubbled up at one end of the pool and swimmers noticed an odor. Swimmers attending the Summer Splash Day Camp may have been exposed to chlorine and acid, fire officials said. Paramedics evaluated 18 children who were complaining of lightheadedness, nausea, and throat burning, according to the Riverside Fire Department. The pool was evacuated, with 79 people removed from the area and into a nearby community center and elsewhere, the department stated in a news release.

At the time, officials said that chlorine did not mix properly before entering the pool.

Hours later, the Riverside Fire Department said they were still investigating the hazmat situation.

'Riverside Parks Department maintenance personnel worked with the Riverside Fire HazMat team to identify what was causing the symptoms,' the statement reads. 'Crews utilized advanced air and water monitoring devices, but all levels were within normal limits. The incident remains under investigation.'

The children were all returned to their parents or guardians.