Millions of Americans were in the path of a major storm on Sunday that caused flash flooding and devastation throughout the middle of the country, The Weather Channel's Chris Warren reports.

Ferocious storms that battered Oklahoma and Missouri, killing 21 over the weekend, lurched eastward Monday, lashing New England and the mid-Atlantic region with harsh winds and heavy rainfall before moving out to sea.

Although the East Coast may be struck by another round of thunder Monday, forecasters at the National Weather Service said the severe storm threat has largely passed, bringing an end to several days of violent weather across a wide swath of the country.

At least 18 people — including six children and three storm chasers — were killed after five twisters attacked the Oklahoma City area Friday evening, terrorizing communities already bludgeoned by lethal storms this spring. Another three people died in Missouri.

Authorities were still searching Monday for five missing people – including three children – from areas around the Oklahoma River, according to Oklahoma City Fire Department Deputy Chief Marc Woodard.

Hospitals in Oklahoma City reported 115 injuries, officials at the Oklahoma State Department of Health said late Sunday, although that number may have increased Monday as officials began to reckon with the devastation wrought by Friday’s swarm of storms.

The twisters hammered the Oklahoma City area just 11 days after a monstrous tornado claimed 24 lives in the suburb of Moore, where power outages were reported Friday during the height of the twisters’ tear through town.

The three fatalities in Missouri were blamed on fierce flooding caused by the punishing hailstorms the wild weather system brought to large parts of the heartland.

"Authorities have confirmed three deaths from high water; those occurred in Lawrence, Miller and Reynolds counties," said a statement from Missouri governor Jay Nixon’s office.

The Weather Channel confirmed at least three tornadoes touched down around St. Louis, badly damaging homes but not causing any fatalities.

Communities on the East Coast were hit with some wild weather conditions Sunday evening, including rough winds and torrential rainfall, but the storm threat had largely subsided by Monday.

“The risk of severe weather has pretty much disappeared on the East Coast,” Bruce Terry, a meteorologist with the National Weather Service, told The Associated Press.

At the height of the Sunday evening assault on the East, over 40,000 homes and businesses were left without power in Vermont, New Hampshire and Maine, according to The Associated Press. But more than half of those structures had electricity restored by Monday.

Washington, D.C., was also pounded by showers and thunderstorms Sunday evening, but the skies above the nation’s capital were mostly clear Monday.

Finally, residents of Anderson County in upstate South Carolina reported possible tornado sightings Sunday, authorities said. At least one house was hit and had its roof torn up, but there were no reported injuries Monday morning, according to County Administrator Rusty Burns.

NBC News' M. Alex Johnson, Janet Shamlian, Aaron Marmelstein and Alastair Jamieson contributed to this report.

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