Twenty people have been killed after powerful storms swamped West Virginia on Thursday night, forcing high water rescues across the state and leaving thousands of customers without power through Friday afternoon, officials said.

The threat of pop-up showers and overflowing rivers was still a concern Friday, and Gov. Earl Ray Tomblin said that search and rescue efforts remained a priority to help people trapped in flooded-out homes and cars. He said 200 National Guard members have been deployed in eight counties with about 300 more authorized to help with ongoing relief.

The storm system dumped 9 inches of rain on parts of West Virginia and trapped 500 people in a shopping center when a bridge washed out. Dozens of other people had to be plucked off rooftops or rescued as waters quickly rose during the deluge.

The heavy rainfall over six to eight hours prompted the National Weather Service to call it a "one-in-a-thousand-year event."

"Please continue to work together and support each other as West Virginians always do," Tomblin pleaded at a news conference where he announced the growing death toll and said about 100 homes have been damaged.

One search for a missing 4-year-old ended in heartbreak after officials said Friday they found his body. The boy — Edward McMillion — had vanished in the rising waters in Ravenswood, Jackson County, which sits along the Ohio River on the Ohio border.

Flooding in Elkview, West Virginia on June 24, 2016.Caleb Smith

At least three other deaths occurred in Kanawha County, while another death occurred in Ohio County in West Virginia's northern panhandle, the governor's office said earlier Friday.

Greenbrier County Sheriff Jan Cahill announced Friday afternoon that two males were also found dead, and called the situation "complete chaos."

Officials did not immediately identify the victims, although local reports said one of those killed in Ohio County was an 8-year-old who had slipped into a creek and was carried away. He later died at a hospital, The Intelligencer reported.

Meanwhile, an elderly man was killed in the floodwaters in Kanawha County, while a woman there was washed away in her vehicle in another incident, NBC affiliate WSAZ reported.

In one dramatic scene in White Sulphur Springs in the state's south, where up to 10 inches of rain fell Thursday, the flood waters pushed a burning home down the Howard Creek.

Natural gas service was shut off in White Sulphur Springs as a precaution, Tomblin said.

"We surely need your prayers because there's a lot of people hurting right now," added Jim Justice, owner of the luxury Greenbrier Resort in White Sulphur Springs, told The Weather Channel.

Forty-four counties, primarily in the southeastern part of the state, were under a state of emergency Thursday night because of flood waters, emergency officials said. Electric utilities initially reported that almost 500,000 customers were without power, although that number dropped to 60,000 by Friday afternoon.

Rivers around the state were reaching historic levels Friday, officials said.

In Clay County, northeast of Charleston, the Elk River rose to over 33 feet, breaking the previous record of 32 feet set in 1888.

Flooding in Elkview, West Virginia on June 24, 2016.Caleb Smith

Kanawha County was especially inundated, and dozens of water rescues were under way in and around Clendenin, where numerous roads were closed by racing flood waters, WSAZ said.

"There's been a ton of areas that have been hit hard" by as much as 7 inches of rain, said Danielle Banks, a meteorologist for The Weather Channel.

Mark Burgess, wife Lisa and parents-in-law Bill and Peggy Lucas were occupying the second floor of their home in Elkview when the waters started rising at around 1:30 am ET.

Mark Burgess told NBC News on Friday morning there was about four feet of water, "but it has stopped going up now."

He said it was the first time that residents have witnessed such flooding. "My mother-in-law has lived here for 80 years, and this never happened before," he said.

"There is nothing you can do," he added. "We can call for help if it's too deep, but it's OK right now."

In neighboring Virginia, thousands were also without power, utilities said.

Record floods were forecast overnight Thursday along the Jackson River in the areas of Covington and Alleghany County. The state Department of Emergency Management urged residents of those areas to be ready to evacuate on short notice.

"It's really bad. There's a raging river," Alleghany County Chief Sheriff's Deputy Matt Bowser said. "There's 500-gallon oil tanks and trash cans and gas cans and tree trunks floating down the river where people's backyards are. You can see people in the second-story windows waiting to be evacuated."

The fierce rain was caused by a "derecho" weather system that spun off at leasteight confirmed tornadoes Wednesday in Ohio and northern Illinois on Wednesday evening and Thursday morning, the NWS said.

The weather service said late Thursday that the system was dissipating rapidly and that many watches and advisories had been canceled.