Howard County Executive Allan Kittleman said the city was flood prone but ones brought by hurricanes paled in comparison to what two hours of rain did Saturday night.
The 6 inches of rain between 7 p.m. and 9 p.m. was the equivalent of a month of normal rainfall, he said.
"We've never seen such devastation in Howard County for over 50 years," he said. "In the past, it has been bad. It has never been close to being this bad ...
"We talked to a lot of people who said they had very little time to get ready -- like ten minutes and all of the sudden it was there," he added. "So that just goes to show you the strength of this, and it also goes to show you that nature is awfully powerful, folks. We are not in control and we have to be ready."
Ellicott City, an affluent unincorporated community of 65,800 people and famous for its historic downtown, is about 12 miles west of Baltimore and in the valley of the Patapsco River, a major waterway flowing to Chesapeake Bay.
Video from the scene shows cars being swept away in roads that look like rivers. "People formed human chains to help out others," said Andy Barth, spokesman for the County Office of Emergency Management.
Courtney Weglein told CNN affiliate WJZ the water got so high so quickly on Saturday night that a man had to be carried to safety.
"It was insane, she said. "It was a flash flood. And I literally came here, I thought we're going to have dinner. Within five minutes, it was panic and my friend was pulling someone out of there, and I've never seen anything like it. ... All I can say is, I'm just happy to be alive."
Kittleman said he would never forget the video of residents forming a human chain to rescue a person trapped in a car in the rising water.
"I will never forget that because that tells me what the people of Ellicott City are like. That tells me that's what people in Howard County are like," he said. "That tells me this is not going to defeat us. We are going to become a stronger, better community, because that is who we are."
Flash flood carried off two people as it ravaged downtown
At least two people died and their bodies were recovered on the Baltimore side of the river, said Howard County Police Chief Gary Gardner. They had both been in cars when the flooding hit.
One of the victims was Jessica Watsula, 35, of Lebanon, Pennsylvania, according to Baltimore County Police. She and her family had been visiting Ellicott City when the flash flood occurred, her brother told police.
They were inside the Watsula's vehicle, when they were swept away by the flood. Everyone in the vehicle managed to get out. All but Watsula was rescued. She was swept away and her body was later found about 200 yards from the Ilchester Bridge.
The other victim was identified as Joseph Blevins, 38, of Windsor Mill, Maryland, police said. He had been in Ellicott City with his girlfriend, when their vehicle was swept away by the water. The woman managed to escape from the car and was eventually rescued, but Blevins was swept away.
Authorities don't know of any other people who are missing, Gardner said.
Firefighters had a busy night, fetching people from the swift water on Main Street and along the Patapsco River, Deputy Fire Chief Frank Rommal said.
"They just went from rescue to rescue," he said
More than 170 damaged cars are being towed to impound lots, Gardner said.
The county has evaluated 200 of the buildings downtown and found that four or five are destroyed, while another 20 to 30 are heavily damaged, said Robert Frances, director of the of the county Department of Inspections, Licenses and Permits.
No firm financial damage estimate is available, but Ryan Miller, director for emergency management in the county, said it would run into the millions of dollars.
Residents and business people have been banned from returning downtown because of the danger, Gardner said. "We are providing security 24-7 in that area to make sure no one goes in those homes or businesses," he said.
Maryland Gov. Larry Hogan has declared a state of emergency in Howard County after surveying the damage Sunday. Lt. Gov. Boyd Rutherford and U.S. Rep Elijah Cummings also came to witness the destruction and start preparing to lobby for federal disaster assistance.
"It looks like a war zone," Kittleman said. "Or as Lt. Gov Rutherford told me, like a set from a disaster movie."
Miller said the water levels have returned to normal but more damage could be on the way. The National Weather Service has forecast more thunderstorms for the low-lying area.
He noted the watershed area was small, with streams converging in a tight area in the city.
"We are watching the weather very closely," he said. "We're watching some storm clouds move in."