HOUSTON -- At least 17 people have died from heat-related illnesses as a heat wave continues baking Texas while spreading into other parts of the southern United States.

In Webb County, southern Texas, medical examiner Dr. Corinne Stern said on Wednesday that 11 residents over the age of 60 have died due to the abnormal heat.

"This is heat like we've not seen here before," said Stern, "Deaths due to heat stroke are ruled as accidents, and accidents, by definition, are preventable deaths. All these deaths could have been prevented."

A 14-year-old boy from Florida died of heat-related fatigue in Big Bend National Park in Texas on Friday, when temperatures there rose to 119 degrees Fahrenheit (48 degrees Celsius), the second-highest mark ever recorded in the state.

Also among the confirmed victims were a 17-year-old hiker in northern Texas, a utility lineman in eastern Texas, a postal worker in Dallas and two residents in the coastal state of Louisiana, local media reported.

A Texas Tribune report on Wednesday said that since the heat wave gripped Texas, at least nine inmates, including two men in their 30s, have died of heart attacks or unknown causes in prisons lacking air conditioning. It has been 11 years since the state's prison system last classified a death as heat-related.

According to Stern, the number of heat-related deaths also rose near the U.S.-Mexico border in the past few weeks.

Five bodies have been found since last week in a desert close to the Mexican border and near human smuggling zones in the Sunland Park area, New Mexico, the El Paso Times reported. It is unknown whether the bodies are those of migrants.

The U.S. National Weather Service forecast temperatures will likely reach 110 to 115 degrees Fahrenheit on Thursday and Friday in Texas as the heat spreads into Arkansas, Louisiana, Kansas and Oklahoma. More than 150 heat records could be broken during the next six days.

Power use in Texas hit an all-time high on Tuesday and is expected to set a new record on Wednesday, said the Electric Reliability Council of Texas, which is in charge of the state's grid.

More than 120 million people in the United States were under various heat safety alerts on Wednesday, according to heat.gov, the web portal for the country's National Integrated Heat Health Information System.

Climate change is widely blamed for causing heat waves to be more intense, longer-lasting and more frequent.