All flights stopped nationwide

The FAA halted all U.S. flights after the attacks on the World Trade Center.

WASHINGTON (CNN) -- The first-ever grounding of all flights in the United States will remain in effect until at least noon EDT Wednesday due to security concerns following Tuesday's hijackings of airliners that were then crashed into buildings, the Federal Aviation Administration said.

The groundstop affects 36,000 to 40,000 flights that take off in the United States daily, as well as general aviation flights. Also, no flights will be allowed to come into the United States from other countries.

"The earliest the national groundstop will be lifted is noon tomorrow. And that's at the very earliest," FAA spokesman Les Dorr said Tuesday.

CONTACT NUMBERS Friends or family members who want more information on the crashes should contact: American Airlines 1-800-245-0999 United Airlines 1-800-932-8555 MORE STORIES Four flights lost; 266 on board     EXTRA INFORMATION Read statements issued by:   American Airlines   United Airlines

He said 1.6 million people fly daily in the United States. Dorr said he didn't think passengers would feel inconvenienced by the stoppage because of the nature of the tragedy.

"Obviously, this is an unprecedented set of events," he said. "I think most people will understand what is going on."

As of 1:30 p.m. EDT, Dorr said, all 40 to 50 domestic airliners that had remained in the air had landed. By about 5 p.m. EDT, the FAA said all international flights that were in the air had landed at West Coast airports.

In what appeared to be coordinated acts of terrorism, two jets crashed into the twin towers of the World Trade Center, collapsing them in clouds of smoke, while another aircraft crashed into the Pentagon.

A fourth aircraft crashed in Pennsylvania.

Senior FBI sources said they are working on the assumption that all four planes were hijacked as part of a terrorist attack.

CNN.COM SPECIAL REPORT  CNN NewsPass Video  Agencies reportedly got hijack tips in 1998  MORE STORIES Intelligence intercept led to Buffalo suspects Report cites warnings before 9/11  EXTRA INFORMATION Timeline: Who Knew What and When? Interactive: Terror Investigation Terror Warnings System Most wanted terrorists What looks suspicious? In-Depth: America Remembers In-Depth: Terror on Tape In-Depth: How prepared is your city?  RESOURCES On the Scene: Barbara Starr: Al Qaeda hunt expands? On the Scene: Peter Bergen: Getting al Qaeda to talk

American Airlines and United Airlines both reported losing two jetliners.

United Airlines has grounded all of its flights worldwide.

Los Angeles International Airport (LAX) was evacuated after the attacks. San Francisco's International Airport was also shut down and evacuated. Boston's Logan International Airport was closed indefinitely, an official said.

Other forms of transportation were affected as well.

Amtrak canceled all train service in the northeast corridor from Boston to Washington, and Greyhound bus company also canceled operations in the northeast and in "select locations" around the country.