At least nine people drowned while swimming in an Arizona river when they became overwhelmed by flash flooding following heavy rains in the drought-stricken region, officials said on Sunday.

Authorities were searching for at least one other missing person after the incident on Saturday afternoon in the Verde River near Payson, about 90 miles (145 km) northeast of Phoenix.

Local media reported at least two children were among the dead. The Gila County Sheriff’s Office did not give details of the deceased or those missing.

A search-and-rescue operation was under way involving sheriff’s deputies and the Arizona Department of Public Safety Ranger Helicopter, as well as members of the local Whispering Pines Fire Department and U.S. Forest Service, Sheriff Adam Shepherd said in a statement.

The National Weather Service in Phoenix said that most of Arizona were under flash flood watch until Monday evening, warning that “heavy rains can produce flash flooding of low lying, normally dry areas such as washes.”