LATEST DEVELOPMENTS

• At least 51 people were killed, including 20 children, according to the Oklahoma medical examiner. Officials said the death toll was expected to rise.
• CNN reports that 40 additional people have died, in an interview with Amy Elliott, chief medical examiner.
• Rescue workers continue to search the rubble of Plaza Towers Elementary School for some two dozen missing children, Oklahoma Lt. Gov. Todd Lamb said.
• Area hospitals reported at least 230 people injured, including at least 45 children.
• The storm laid waste to scores of buildings in Moore, a community of 41,000 people south of Oklahoma City.
• The National Weather Service issued an initial finding that the tornado was an EF4 on the enhanced Fujita scale, the second most-powerful type of twister.
• President Obama declared a major disaster in Oklahoma and ordered federal aid to supplement state and local recovery efforts. Aid includes grants for temporary housing, home repairs, uninsured property losses and other recovery efforts.
• Oklahoma City Police Capt. Dexter Nelson said downed power lines and open gas lines pose a risk in the aftermath of the system.
• Oklahoma organizations are collecting financial donations for tornado victims.
KFOR-TV in Oklahoma City reports that Moore has no running water. Crews are working to get it fixed.

ELEMENTARY SCHOOL
Several students were pulled alive from the destroyed Plaza Towers Elementary School. Rescue workers passed the survivors down a human chain to the triage center in the parking lot.
James Rushing, who lives across the street from the school, heard reports of the approaching tornado and ran to the school, where his 5-year-old foster son, Aiden, attends classes. Rushing believed he would be safer there.

“About two minutes after I got there, the school started coming apart,” he said.

Students were placed in the restroom.

RESCUE EFFORTS
• Volunteers and first responders continue to search debris for survivors. Search and rescue efforts were to continue throughout the night.
• Oklahoma Gov. Mary Fallin deployed 80 National Guard members to assist with search-and-rescue operations and activated extra highway patrol officers.
• President Barack Obama spoke with Fallin, offering the nation’s help and a direct line to his office.
• Officials in Joplin, Mo., have assembled a team of about a dozen police and firefighters they are sending to Moore. Joplin was devastated by a tornado two years ago that killed 158 people and injured hundreds more.

How to help Oklahoma tornado victims

AS IT HAPPENED
The town of Moore, population about 50,000, was devastated with debris everywhere, street signs gone, lights out and houses completely obliterated.

Another elementary school and a hospital were among the buildings leveled.

“We thought we died because we were inside the cellar. … It ripped open the door and just glass and debris started slamming on us, and we thought we were dead to be honest,” survivor Ricky Stover said while surveying the devastated remains of his home.

Cyndi Christopher was at work and went to pick up her son from day care when she heard the storm warning. After taking her son home, she was forced to flee when she noticed the storm was coming their way.

“I drove as fast as I could, and I outran the storm,” Christopher said.

In video footage, the dark funnel cloud moved slowly across the landscape for more than half an hour, scattering shards of wood, pieces of insulation, shingles and glass over the streets.

“Hearts are broken” for parents looking for their children, Fallin told a news conference.

Block after block of Moore lay in ruins. Cars and trucks were left crumpled.

The focus quickly turned to Plaza Towers Elementary School, where the storm tore off the roof, knocked down walls and turned the playground into a mass of twisted plastic and metal.

After the tornado passed, Tiffany Thronesberry said she got a panicked call from her mother, Barbara Jarrell.

“I got a phone call from her screaming, ‘Help, help! I can’t breathe. My house is on top of me!’” Thronesberry said. She hurried to her mother’s house, where first responders had already pulled her out with cuts and bruises.

Briarwood Elementary School, which also stood in the storm’s path, was all but destroyed. On the first floor, sections of walls had been peeled away, affording clear views into the building, while in other areas, cars hurled by the storm winds were lodged in the walls.

Across the street, people picked through the remains of their homes, looking for any possessions they might salvage.

The University of Oklahoma Medical Center had received at least 20 injured. St. Anthony Healthplex South in Oklahoma City said it received four patients and Midwest Regional received four.

“They (injured) are coming in minute by minute,” said Integris Southwest Medical Center spokeswoman Brooke Cayot. Of the 19 injured there, seven were in critical condition, seven serious and five listed as fair or good, Cayot said.

Gallery: Tornadoes rip through Oklahoma town

Moore Medical Center sustained significant damage.

“The whole city looks like a debris field,” Glenn Lewis, the mayor of Moore, told NBC.

“It looks like we have lost our hospital. I drove by there a while ago and it’s pretty much destroyed,” Lewis said.

Fire, rescue and emergency medical teams from across the state converged on Moore, and members of the National Guard were on the scene, said Terri Watkins, a spokeswoman for the Oklahoma Department of Emergency Management.

“They are going to going to go house to house, building to building to determine whether anyone is trapped,” Watkins said. “If anyone is trapped we want to begin pulling them out as quickly as possible.”

MONDAY’S TORNADO
• 200 mph – Less than 1 percent of all tornadoes reach such wind speed.
• The tornado first touched down in Newcastle, then hit Plaza Towers Elementary School, 10 miles away.
• The National Weather Service estimated that the tornado reached up to a half-mile wide and was an EF-4 on the enhanced five-point Fujita scale, the second most powerful type of twister.
• The National Weather Service Storm Prediction Center provided the town with a warning 16 minutes before the tornado touched down at 3:01 p.m. local time (4.01 p.m. EDT), which is greater than the average eight to 10 minutes of warning.
• The notice was upgraded to emergency warning with “heightened language” at 2:56 p.m., or five minutes before the tornado touched down.
• The massive twister struck at the height of tornado season.

1999 TORNADO
Monday’s powerful tornado loosely followed the path of a killer twister that slammed the region in May 1999, killing more than 40 people and destroying thousands of homes. That tornado ranked as an EF5, meaning it had winds over 200 mph.

Kelsey Angle, a weather service meteorologist in Kansas City, Mo., said it’s unusual for two such powerful tornadoes to track roughly the same path. The 1999 storm had winds clocked at 300 mph.

The 1999 tornado ranks as the third-costliest tornado in U.S. history, having caused more than $1 billion in damage at the time, or more than $1.3 billion in today’s dollars. Only the devastating Joplin and Tuscaloosa tornadoes in 2011 were more costly.

TORNADO STRENGTH
Monday’s tornado was preliminarily rated EF4, the second most powerful category on the Enhanced Fujita Scale, with the 1999 tornado rated EF5, the strongest.

The Enhanced Fujita Scale is an updated version of an earlier chart to measure the ferocity of tornadoes published in 1971 by a University of Chicago professor.

Tornadoes: Separating fact from fiction

In rating the intensity of tornadoes, meteorologists and other experts study damage on the ground along with wind speeds and other data to rate twisters between EF0 and EF5.

The scale is as follows:
• An EF5 tornado, capable of causing incredible damage, is characterized by three-second wind gusts between 200 and 234 mph and can knock strong frame houses off their foundations, according to the National Oceanic and Atmospheric Administration (NOAA). Automobile-sized missiles can fly through the air in excess of 109 yards, and trees can be stripped of their barked. The devastating tornado that struck the town of Joplin on May 22, 2011, killing 161 people, was an EF5 tornado.
• An EF4 tornado can cause devastating damage and is capable of leveling well-constructed homes and blowing away those with weak foundations. Cars can be thrown and large missiles generated. Such a twister includes wind gusts of between 168 and 199 mph.
• An EF3 tornado can cause severe damage, tearing roofs and walls off well-constructed homes, uprooting trees, and lifting cars of the ground and throwing them. It has wind gusts of between 138 and 167 mph. Several EF3 tornadoes have been recorded in the United States this year, including one that touched down in Kemper County, Miss., in April, killing one person and injuring four.
• An EF2 tornado can cause considerable damage, tearing roofs off frame homes, demolishing mobile homes, overturning boxcars and snapping or uprooting large trees. Such a tornado is characterized by wind gusts between 110 and 137 mph. A number of EF2 tornadoes have struck the United States so far during 2013, in some cases causing injuries or substantial damage.
• An EF1 tornado can cause moderate damage, with wind gusts of between 86 and 109 mph. Damage caused in such twisters includes surfaces peeled off roofs, mobile homes pushed off foundations or overturned and cars blown off roads.
• An EF0 tornado causes light damage with wind gusts of between 65 and 85 mph. The impact can include damage to chimneys, branches broken off trees and shallow-rooted trees being knocked over.
The Associated Press and Reuters contributed to this story.