Richard Rowe / Reuters A tornado near El Reno, Okla., on May 31.

The tornado that struck El Reno, Okla., on Friday night is believed to be the widest on record in the United States at 2.6 miles across, the National Weather Service reported on Tuesday.

The May 31 twister beat the previous record, a 2.5-mile wide storm that struck Hallam, Neb., on May 22, 2004.

The Friday tornado and its resulting flooding claimed 18 lives, including six children and a dozen adults, the Oklahoma Medical Examiner reported on Tuesday. Six of the victims remain unidentified. Area hospitals treated 115 injuries after Friday’s storm, the Oklahoma Department of Emergency Management said.

The tornado’s width was measured by a mobile radar unit as the storm passed south of El Reno near Highway 81, the NWS said. The measurement only includes the tornado itself, and not winds surrounding the storm.

The tornado was also upgraded to an EF-5 classification, the most forceful category, according to the NWS.

El Reno is about 25 miles west of Oklahoma City.

The massive tornado struck less than two weeks after a tornado ripped through nearby Moore, Okla., leaving 24 people dead.

Governor Mary Fallin declared a state of emergency in 41 counties as the storm-battered region recovered on Tuesday. Search and recovery efforts continued for victims who went missing after being swept away by Oklahoma River floodwaters, NBC News affiliate KFOR reported.

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