El Reno, Oklahoma

Two pickup trucks are pictured stuck in high water along NW 23rd Street in El Reno, Oklahoma. (Bill Waugh, Reuters / June 1, 2013)

A tornado that swept through Oklahoma on Friday was the widest tornado in American history, the National Weather Service said Tuesday.

The El Reno, Okla., tornado scraped out a damage path up to 2.6 miles wide and 16.2 miles long, a swath at points wider and longer than Manhattan. The storm broke the record held by a 2.5-mile-wide Hallam, Neb., twister.

"It was amazing and something that's extremely rare," Howard Bluestein, professor of meteorology at the University of Oklahoma, said of the storm's strength -- which now presents a puzzle for researchers whose solution could help mean the difference between life and death.

The human aftermath left by Friday's twister was painfully apparent, with at least 18 people killed in the latest massive tornado to carve through Oklahoma this spring.

The storm itself, however, remains much more of a mystery.

Researchers don't know why the twister got as big and powerful as it did, and its strength wasn't immediately apparent as it scoured a rural area, leaving few of the physical clues that help determine wind speeds.

“If you look at a wheat field, how do you know how strong the wind was?" Bluestein said, referring to the technique of using ground damage to measure storm strength.

The twister was originally rated an EF-3 before further measurements boosted the storm's rating to EF-5, the highest possible.

The huge funnel that tore around El Reno was also made up of multiple smaller tornadoes rotating like horses around a merry-go-round, Bluestein said. But what gave them their strength?

“That is something that will be the subject of research for some time to come," said Forrest Mitchell, the observations program leader for the National Weather Service's office in Norman, Okla.

“We have a lot to learn to find out what controls the intensity of a tornado,” Bluestein said. “You can’t tell why, on some days, you’ll get a huge monster tornado like on Friday, while other days they’ll have relatively weak tornadoes or no tornadoes at all.”

And the destruction that came with Friday's storm also left behind a trove of data that could give the kind of precision to weather service forecasts that could save lives.

"Is there a particular balance of elements that are necessary for such a large tornado to form?" said Mitchell. "Is there a threshold that we can determine in the future -- that once we’ve reached that particular threshold, we can forecast with confidence that a strong or violent tornado is highly probable with a particular thunderstorm?"

Mitchell added, "The goal of all the research is to add that additional layer of credibility to our report so that people … will take shelter during a deadly storm.”

ALSO:

Mississippi River to crest above flood stage today

James Holmes' notebook to be turned over to prosecutors

Judge allows plea of not guilty by insanity for James Holmes