The weather service said NWS meteorologists and researchers from the University of Oklahoma and the Center for Severe Weather Research determined the Friday storm reached EF5 status, the most powerful level of tornado with winds exceeding 200 mph.
"In addition, the width of [the] tornado was measured by the mobile radar data to be 2.6 miles after the tornado passed east of U.S. Highway 81 south of El Reno," the NWS said in a statement. "This width is the width of the tornado itself and does not include the damaging straight-line winds near the tornado as determined by the high-resolution mobile radar data.
"The 2.6-mile tornado path width is believed to be the widest tornado on record in the United States."
The twister stayed on the ground for 40 minutes, covering 16.2 miles. The El Reno tornado was part of a series of tornadoes, severe storms and subsequent flooding that killed at least 16 people and left five others still missing.
Notable deaths of 2014 [PHOTOS]