CHICAGO : Three veteran storm chasers were among the 14 people killed when powerful tornadoes tore through the US state of Oklahoma, officials said Monday.

Tim Samaras, his son Paul and their storm-chasing partner Carl Young died Friday in a twister in El Reno, west of Oklahoma City. They were the first storm researchers ever killed while chasing tornadoes, media reports said, citing the National Weather Service's Storm Prediction Center in Norman, Oklahoma.

"They all unfortunately passed away but doing what they LOVED," the elder Samaras's brother Jim Samaras said on his Facebook page. Samaras's instruments are said to have offered the first-ever glimpse inside a tornado, and his Tactical Weather Instrumented Sampling in Tornadoes Experiment, or TWISTEX, aimed to learn more about the storms in order to help increase the lead time for warnings.

"We still don't know why some thunderstorms create tornadoes while others don't," Samaras told National Geographic in one of his last interviews.

"We're trying to collect as many observations as possible, both from outside and from the inside."

A series of tornadoes battered Oklahoma with high winds, heavy rain and large hail, killing 14 people, including five children, in a state already reeling from a monster twister that claimed two dozen lives last month.

The Oklahoma state chief medical examiner confirmed the toll, which included the three storm chasers and six unidentified individuals.

Debris from the chasers' vehicle was strewn across about half a mile (0.8 kilometers), Canadian County Undersheriff Chris West told AFP.

One of the bodies was recovered from the vehicle, while the two others were found about a quarter of a mile in either direction.

Crews hauled away a badly mangled white truck with its windows smashed and its body crushed and twisted almost beyond recognition.

"Tim was a courageous and brilliant scientist who fearlessly pursued tornadoes and lightning in the field in an effort to better understand these phenomena," National Geographic executive vice president Terry Garcia said.

The Washington-based institution had provided 18 grants to Samaras for his research.

It said he developed his interest in twisters after watching the film "The Wizard of Oz" when he was just six years old. The movie begins with a tornado sweeping heroine Dorothy and her dog Toto away to the magical Land of Oz.

"Though we sometimes take it for granted, Tim's death is a stark reminder of the risks encountered regularly by the men and women who work for us," Garcia said in a statement.

Samaras developed probes to measure the environment inside tornadoes. Researchers had to place the devices in the path of the storm and then escape before being swept away.

He measured the lowest barometric pressure drop ever recorded (100 millibars) at a tornado's center, saying that was equivalent to "stepping into an elevator and hurtling up 1,000 feet (305 meters) in 10 seconds."

Samaras was also a star on the Discovery Channel's show "Storm Chasers," which ended last year.

His brother Jim Samaras said the storm chaser "looked at tornadoes not for the spotlight of TV, but for the scientific aspect."

"At the end of the day, he wanted to save lives and he gave the ultimate sacrifice for that," he told The Denver Post.

The storm-chaser's final message on the social media site Twitter warned: "Storms now initiating south of Watonga along triple point. Dangerous day ahead for OK (Oklahoma) -- stay weather savvy!"

Despite the big risks, Samaras was known to be very cautious.

"He knew where not to be and in this case the tornado took a clear turn toward them," Jim Samaras said.

Weather Channel anchor and meteorologist Mike Bettes had a close call himself in Oklahoma's storms, and described them as highly unpredictable.

The tornado swept up the truck he was traveling in with a crew, throwing it 200 yards (meters) into a field and smashing it to the ground.

"I think this was just an erratic tornado. I think the size of it and the speed of it changed very, very quickly," Bettes told CNN.