The heaviest snow of the season blanketed the nation's capital and surrounding area Monday, making a mess of the roadways and giving many school pupils a day off.

Most major thoroughfares were plowed by midmorning, but some lanes and road markings remained covered with slush. Conditions were especially treacherous on side streets.

"We've been trying to hit all 1,100 miles of roads," focusing first on the main arteries, said Dan Tangherlini, director of the District of Columbia's Department of Transportation.

Even as the snow began to taper off, leaving as much as seven inches in some areas, forecasters warned of a potentially bigger problem to come -- ice. The National Weather Service (search) extended a winter storm warning through Monday night, saying the snow was expected to gradually change over to sleet and freezing rain. Ice accumulations of about one quarter of an inch were possible.

A snow emergency in the District of Columbia remained in effect Monday morning, banning parking on several major streets. About 600 vehicles had been towed by sunrise, Tangherlini said.

Many schools were closed in the city and neighboring Maryland and Virginia, but the federal government remained open, with an unscheduled leave policy for employees. Maintenance workers armed with snow blowers, shovels and salt spreaders cleared sidewalks around major federal buildings before dawn.

The wintry weather didn't prevent the subway system rail from operating on a normal schedule during the morning commute, although there were some delays. Bus service was limited only to snow emergency routes.

Still, it appeared that many people decided to stay home.

"On Metrorail through 8 a.m. (EST) we have carried 64,919 people," said Metro spokesman Steven Taubenkibel. On a normal workday, the transit service typically carries about 120,000 passengers.

The district, Maryland and Virginia mobilized hundreds pieces of equipment Sunday night to handle the storm -- including, snow plows, salt spreaders and tow trucks. Although crews across the region worked to make streets passable, officials conceded that the mixture of chemicals did not work well because temperatures remained below 26 degrees.

"We could get hit around the afternoon rush hour with sleet and freezing rain, so we're going to have to go 24 hours a day until this is over," said MDOT Secretary Robert Flanagan.