CHICAGO (Reuters) – A monster winter storm stretching from New Mexico to Maine laid down a sheet of ice on the Plains and lower Midwest on Tuesday, turning to snow as it crept north, and forecasters said the worst was yet to come.

"We're looking at heavy snow from the Rockies to New England," said Pat Slattery, National Weather Service spokesman.

The storm was expected to affect as much as a third of the U.S. population, from severe thunderstorms in the Southeast and ice-coated roads in Texas to blizzard conditions and up to 2 feet of snow in Chicago. Several regions were experiencing bitter cold, including Denver, where mid-afternoon readings fell to six degrees below zero.

In Oklahoma City, more accustomed to tornadoes than blizzards, homeowner Debra Thomas, 53, said the entire town was largely snowbound as gale-force wind gusts and a foot of snowfall created white-out conditions and large drifts.

"I'm looking out my door here, and I've got at least 4 feet, maybe 5 feet, up against the windows of our house," she told Reuters by telephone. "It's very pretty, but not the kind of thing you go out in."

The Dallas-Fort Worth international airport closed for 2 1/2 hours on Tuesday just as thousands of football fans began arriving in the city for the Super Bowl game on Sunday. And the storm forced postponement of a National Hockey League game in St. Louis between the Blues and the Colorado Avalanche.

About 200 National Guard troops were called out to assist local law enforcement in the St. Louis area.

Major U.S. airlines said they had canceled more than 5,400 flights ahead of the storm. More than 1,200 of those flights were in and out of Chicago-area airports, major airline hubs.

One of the hardest-hit cities on Tuesday was Kansas City, where a municipal state of emergency was declared and snow plows were pulled from runways at the international airport in late afternoon due to blowing snow and low visibility.

Statewide emergencies were declared in Missouri, Oklahoma, Illinois and Wisconsin. White-out conditions were reported on Interstate 44 between Oklahoma and Missouri, closing part of that highway, as well as Interstate 70 in central Missouri.

"Everyone should stay inside today and not drive," Missouri Governor Jay Nixon said on local television.

Weather Service meteorologist David Gaede said: "There's not much traffic moving in Missouri tonight."

"Black ice" conditions prompted Nebraska state police to shut down Interstate 80 and I-680 in Omaha at the start of the evening rush hour.

Storm-related outages knocked out power to more than 100,000 homes and businesses in at least eight states, including Texas, Louisiana, Arkansas, Missouri, Kentucky, Indiana, Illinois and Ohio.

Once the snow lets up Wednesday, affected areas will remain stuck in a deep freeze through the end of the week, with single digit temperatures in the daytime and "really dangerous wind chills," Slattery said.

Schools were closed all along the storm's path, from Denver through Ohio. Chicago public schools, famous for staying open no matter the weather, stayed open Tuesday but were ordered closed on Wednesday.

The snow began coming down hard in Chicago by 2:30 p.m. In the morning, residents were already setting out lawn chairs and plastic buckets to stake out parking spots in advance of the storm. The city readied all 274 of its snow plows for service, along with 120 garbage trucks fitted with plow attachments.

The National Weather Service said Chicago could get its biggest blizzard in more than 40 years, accompanied by winds of more than 40 miles per hour and plunging temperatures.

At the CME Group's downtown trading floor in Chicago, many traders who buy and sell agricultural commodities, financial contracts and options were planning to stay downtown overnight, storing suitcases in the coat room. But trading was not expected to be affected.

Bob Margherio, owner of Mac Do It Best Hardware in Webster Groves, Missouri, said he had run out of ice-melt spread, snow blowers, generators, and flashlights and had nearly exhausted his inventory of batteries.

In New York City, forecasters said the wintry mix of snow, sleet and freezing rain would last into Wednesday as the storm moved in from the Midwest.

"It's far from a picturesque scene. It's an icy, wet mess," said meteorologist John Davitt of local NY1 television.

The storm was expected to wreak havoc on farm operations in the Plains states, threatening the dormant winter wheat crop, cattle herds, and grain deliveries.

(Additional reporting by Corrie MacLaggan, Carey Gillam, Doris Frankel, Bob Burgdorfer, Steve Gorman, David Hendee, Ann Saphir, Bruce Olson, Wendell Marsh, Lauren Keiper, Ben Fenwick, Ellen Wulfhorst and Keith Coffman; Editing by Steve Gorman and Jerry Norton)

WASHINGTON (Reuters) – A classified U.S. document obtained by the anti-secrecy group WikiLeaks shows three previously undisclosed participants in the September 11, 2001 plot, The Washington Post reported on Tuesday.

The three Qatari men arrived in the United States on August 15, 2001, conducted surveillance of targets and left the country on the eve of the September 11 attacks, according to the leaked U.S. diplomatic cable.

The three men "visited the World Trade Center, the Statue of Liberty, the White House, and various areas in Virginia" before flying on to Los Angeles, according to the leaked document.

A U.S. official, speaking on the condition of anonymity, said the three Qataris were "looked at" within days of the attacks and that investigators concluded they could not be charged, The Washington Post reported.

"There is no manhunt," the official was quoted as saying. "There is no active case. They were looked at, but it washed out," he was quoted as saying, downplaying a report by Britain's The Daily Telegraph, which said the FBI has launched a manhunt for the previously unknown team of men suspected to be part of the attacks.

The CIA and the FBI declined to comment on the cable, the Post said.

The report said the three Qataris were part of a 2002 FBI list of people whom authorities wanted to interview about the September 11 attacks.

After the men left the East Coast, they stayed at a hotel near the Los Angeles airport. Hotel staff later told investigators the men had "pilot-type" uniforms and computer printouts listing pilot names, airlines, flight numbers and flight times, the cable said.

The men were scheduled to fly to Washington on September 10, 2001, on the plane that was hijacked the next day and flown into the Pentagon. Instead, they flew to London and then on to Qatar on September 13, according to the report.

The three Qataris were part of a 2002 FBI list of people whom authorities wanted to interview about the September 11 attacks, the Post said .

The leaked cable was sent on February 11, 2010, from the U.S. Embassy in Doha, Qatar to various agencies in Washington, recommending that a man identified as Mohamed al-Mansoori be added to a government watch list, the Post.

"He is suspected of aiding people who entered the U.S. before the attacks to conduct surveillance of possible targets and providing other support to the hijackers," the cable said.

Mansoori, from the United Arab Emirates, lived in Long Beach, California in September 2001, the Post said.

(Reporting by JoAnne Allen, editing by Christopher Wilson)

CHICAGO/ATLANTA (Reuters) – Airlines canceled thousands of flights on Tuesday ahead of a blizzard that has promised near-record snowfall in the U.S. Plains and the central Midwest.

The storm was expected to stretch across 2,000 miles -- dumping up to 2 feet of snow in the central and upper Midwest, as well as ice and sleet in the southern Midwest.

The largest U.S. airlines -- United Continental, Delta Air Lines, AMR Corp's American Airlines, Southwest Airlines and US Airways Group -- had scrapped about 5,400 flights by Tuesday afternoon.

"We are currently looking at approximately 1,900 flights canceled system wide," American Airlines spokesman Tim Smith said. "That's about 55 percent of our total operations."

United Continental, formed from a merger of United and Continental airlines, had canceled 1,450 flights; Delta had canceled 625 flights, US Airways said it had scratched about 690 flights, and Southwest said it eliminated 700 flights through Wednesday afternoon.

"We are proactively reaching out to customers to advise them of rebooking options, and hundreds of additional agents have been on duty in reservations since Sunday," said United spokesman Charles Hobart.

In Dallas, icy conditions that are part of the same weather system briefly closed Dallas/Fort Worth International Airport, a hub for American Airlines, on Tuesday morning. The city's Love Field Airport, which is primarily served by Southwest, was also closed while crews cleared snow and ice and was expected to reopen later in the day, assistant director Kenneth Gwyn said.

David Magana, public affairs manager at DFW Airport, said one runway was reopened at the airport on Tuesday, and ice was being cleared from six others.

Magana said Dallas-Fort Worth Airport had 300 departures canceled so far for the day; the airport normally has about 850 daily departures.

"We've been plowing runways and treating them," Magana said. "Roadways around the airport are still fairly icy as well."

Unpredictable winter weather is a yearly nuisance for airlines and can result in a hit to quarterly earnings. But early flight cancellations can help airlines avoid stranding their passengers at airports and on grounded planes.

"Airlines will proactively cancel flights in order to minimize the impact on customers and employees, and ensure that their operations can return to normal as quickly as possible following weather events," the Air Transport Association industry group said in a statement.

In recent years, both JetBlue Airways and American Airlines suffered public relations nightmares when harsh weather forced last-minute cancellations and left passengers stuck at airports.

Last year, the U.S. Transportation Department approved an airline fine of up to $27,500 per passenger for tarmac delays that exceed three hours. Passengers could be let off a plane, if they made that request.

"They're canceling ahead of time," said Terry Trippler, owner of travel website Airlinerulestoknow.com. "They can lose the revenue without going further in the hole.

"They're getting smart. They've figured it out," he said.

Airline shares were broadly higher in afternoon trading with the Arca airline index up 1.4 percent, roughly in line with broad gains for stocks as measured by the S&P 500 index.

AMR shares were up 1.7 percent to $7.17 on the New York Stock Exchange and Delta stock rose 1.5 percent to $11.85.

United Continental shares were down 0.3 percent to $25.32, but Southwest was 0.3 percent higher at $11.89.

(Additional reporting by Christine Stebbins in Chicago, editing by Maureen Bavdek and Tim Dobbyn)