A “complicated and active” storm system swept across the Eastern United States on Monday evening, delivering widespread thunderstorms that killed at least two people, grounded thousands of flights and left more than a million homes and businesses without power.

The line of storms barreled through a stretch from Georgia to New York, downing power lines, sending trees crashing into homes and tearing roofs from buildings, according to preliminary reports from the National Weather Service.

The Federal Aviation Administration ordered airports to ground flights along a busy travel corridor that links major U.S. cities, including Atlanta, New York and Washington, leading to thousands of flights being delayed or canceled.

At least one tornado was confirmed, just after 5:30 p.m. in the village of McGraw, about 30 miles south of Syracuse, N.Y. The Weather Service said that mobile homes in the village of around 1,000 people would likely be “damaged or destroyed.” The extent of the damage was not immediately clear on Monday night.

In Florence, Ala., a 28-year-old man died after he was struck by lightning in a parking lot in the city, about 60 miles west of Huntsville, local police said. And in Anderson, S.C., a 15-year-old boy was killed when a large tree fell and struck him, according to local fire officials.

In Pennsylvania, a person was injured when a tree fell on the car they were in, according to the preliminary reports.

Some of the worst-hit areas were along the Mason-Dixon line and the southern Appalachians, said David Roth, a meteorologist with the National Weather Service.

He cautioned that while the line of storms, known as a bow echo, had moved off the New Jersey coast, there was still a risk of “excessive rainfall” overnight into Tuesday across parts of upstate New York and Vermont. There, he said, “the heavy rainfall may just be beginning,” noting that the region could expect several inches.

In Cambridge, Md., several inches of rain caused flash flooding that stranded more than a dozen people in their cars on deluged roads, Chief Justin Todd of the Cambridge Police Department wrote by email. No injuries or deaths had been reported, he said, noting that several streets were closed as the police worked with local officials to get debris cleared from the roads.

Rob Kramer, Jr., a Dorchester County councilman, said that while the water was receding, “several roads” remained flooded.

As of around 11:45 p.m. on Monday, more than 600,000 homes and businesses across the Eastern United States remained without power, according to poweroutage.us.

Jesus Jiménez and Lauren McCarthy contributed reporting.