Harnessing the power of four commercial airplanes, a sophisticated but unknown group of terrorists killed thousands while flattening three of the U.S.’ most prominent structures.

Two of the hijacked planes were United Airlines flights, while the other two were from American Airlines.

“The thoughts of everyone at United are with the passengers and crew of these flights,” United Airlines said in a written statement. “Our prayers are also with everyone on the ground who may have been involved in today’s tragic events.”

Al Becker, American Airlines spokesman, shared these sentiments.

“We are horrified by these tragic events,” he said in a written statement.

American Flight 11 and United flight 175, both Boeing 767s flying from Boston to Los Angeles, each crashed into a New York World Trade Center tower. Later, both towers crumbled.

American Flight 77, a Boeing 757 flying from Washington, D.C. to Los Angeles, crashed into the Pentagon military headquarters in Washington, D.C. United Flight 93, a 757 flying from Newark, N.J. to San Francisco, crashed into a field near Johnston, Pa.

While too early to determine the number of fatalities, it is nearly certain all 266 people on all four flights perished.

The total number of civilian fatalities – those in the structures, rescue workers and people on the street – could be in the tens of thousands.

The planes were travelling between 200 and 250 miles per hour at impact, estimated University aerospace engineering and mechanics professor Gary Balas.

“The amount of energy and the plane flying at that speed as well as the fuel is just devastating,” Balas said.

According to the Boeing Web site, the Boeing 757 can hold 11,276 gallons of jet fuel, while the 767 holds 23,980 gallons. Jet fuel is less explosive than gasoline but is still capable of producing a 2,000 degree Fahrenheit inferno inside the World Trade Center towers – hot enough to melt their steel support girders, said Lanny Schmidt a University professor of chemical engineering and materials science.

Structural engineer and University associate architecture professor Bruno Franck said only highly specialized buildings such as nuclear power plants could withstand such an impact, which crumpled both towers less than an hour after the planes struck.

Buildings, Franck said, are not designed to withstand impacts anything like plane crashes. He said he was amazed the towers stood as long as they did. As a result of this event, engineers will likely have to reconsider how they build future structures.

There are usually 4,000 flights in the air at one time, but by noon central time Tuesday, the Federal Aviation Administration had grounded all commercial flights in the U.S. – an unprecedented step.

Balas said at least one hijacker in each plane must have been a pilot. It takes skill to line up a plane to crash into a specific target.

“Whoever was on those planes, there had to be a pilot who was an accomplice,” he said. “I seriously doubt any pilot, even with a gun to his head, would run into a building.”

Once a plane is in the air, only the pilots can control the flight path.

“There is nothing you can do once someone has a plane under their control,” Balas said. “They can run into whatever they want.”