The deadly tornado that struck near Oklahoma City at the end of May had a record-breaking width of 2.6 miles and was the second top-of-the-scale EF5 twister to hit the area in less than two weeks, the National Weather Service said.

The weather service initially rated the massive tornado on May 31 as an EF3, but it upgraded the ranking after surveying damage from the twister, which along with subsequent flooding killed at least 20 people, including three professional storm chasers. The weather service determined that the storm had winds reaching 295 mph.

Oklahoma City area saw two of the extremely rare EF5 tornadoes in only 11 days. The other hit the suburb of Moore on May 20, killing 24 people and causing widespread damage. In 1999, Moore was hit by another EF5 with the strongest winds ever measured on Earth: 302 mph.

When the winds were at their most powerful on May 31, no structures were nearby, said chief warning coordination meteorologist Rick Smith with the weather service.

A two-and-a-half-mile wide tornado would not look like a tornado to a lot of people.

“Any house would have been completely swept clean on the foundation. That’s just my speculation,” Smith said. “We’re looking at extremes … in the rare EF5 category. This in the super rare category because we don’t deal with things like this often.”

Smith said the storm’s wide path would have made it hard to recognize up close.

“A two-and-a-half-mile wide tornado would not look like a tornado to a lot of people,” Smith said.