A storm that struck at least five US states on Friday, killing at least 80 people, was one of the worst to hit the country since the 1920s. Many people are still unaccounted for and the toll is expected to rise.
If confirmed as a single tornado, it would, at 370 kilometres, be the longest continuous twister in American history.
The US National Weather Service said it did not know whether one or several tornadoes had occurred.
If there was only one, it may be classed as an EF5, the most violent category and would be the first in the country since 2013, the US National Weather Service said.
The Enhanced Fujita scale is based on the degree of damage caused, not a measurement of wind speed, and was devised by meteorologist Ted Fujita.
EF5 tornadoes have wind speeds in excess of 320 kilometres per hour.
The destruction caused by the immensely powerful storm was visible in several US states on Saturday.
In Earlington, Kentucky, a freight train was blown over. Freight trains’ locomotives can weigh in excess of 100,000 kilograms, and some models are almost twice as heavy.
Thousands of homes in the state were left without power.
The storms may be the state’s deadliest since 1974, said Michael Dossett, director of Kentucky Emergency Management.
He said its length may exceed that of the deadly storm that hit southern and Midwest states in 1925.
“Rescues and search efforts are ongoing,” he said during a press briefing.
Biden mobilises tornado assistance
On Saturday, Kentucky Governor Andy Beshear declared a state of emergency and activated the National Guard in the aftermath of the storms. President Joe Biden approved an emergency declaration for Kentucky, mobilising federal assistance efforts for 15 counties.
Mr Biden said he’s monitoring the situation “very closely” and also spoke with the governors of Arkansas, Illinois, Missouri and Tennessee. He said if states beyond Kentucky seek federal disaster declarations, he stands ready to approve them.
“The federal government will do everything — everything — it can possibly do to help,” Mr Biden said at a news conference Saturday in Delaware. “We’re going to provide everything that’s needed.”
Mr Beshear said 110 people had been in a candle factory in Graves County when a tornado hit, and only 40 had been rescued. CNN reported that no one had been found since 3 am Saturday.
“It would be a miracle if anyone else is found alive,” Mr Beshear said in an afternoon briefing, before darkness fell on the first day of rescue efforts.
Mr Biden’s declaration authorises the Federal Emergency Management Agency to coordinate relief efforts in the stricken parts of Kentucky.
The National Weather Service warned that nighttime tornadoes “are particularly dangerous” since people may not be listening for severe weather warnings. Unlike hurricanes, the path of a tornado can’t be easily tracked in advance so giving timely evacuation orders is often unfeasible given their swift and often erratic path.
Mr Biden said questions will likely be raised about the advance warning of the storms.
“One of the questions that’s going to be raised, I’m confident, is: What warning was there? And was it strong enough? And was it heeded?” Mr Biden said.
Agencies contributed to this report.