Authorities said they had no immediate reports of fatalities or injuries Sunday.
In addition to the flooding in Ellicott City, flooding was also reported elsewhere in Howard County and in neighboring Baltimore City and county.
Authorities said Ellicott City, on the west bank of Maryland's Patapsco River, is prone to periodic flooding.
A friend of Hermond's told WTOP that the veteran had been in a restaurant in downtown Ellicott City on Sunday; witnesses said he was swept away after trying to help people.
Videos posted to social media show high water rushing down streets.
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Hogan said he is heading to the scene.
In July 2016, Ellicott City received 6.6 inches (17 centimeters) of rain over a two- to three-hour period. On Sunday, the community received almost 8 inches (20 centimeters) of rain over a 6-hour period, but most of it fell during an intense, 3-hour period, Muccilli said.
Gov. Larry Hogan (R) declared a state of emergency in Howard County as a major front drenched the Baltimore region, dumping some 7.5 inches of rain on the town in just eight hours. "This was a true flash flood".
As the flood waters receded late on Sunday, officials were just beginning the grim task of assessing the destruction.
"If you look at the devastation and the damage, I would certainly say it's worse than 2016", he said. The Howard County Fire & EMS agency tweeted that water was above the first floor of some buildings.
The newspaper reports that the swirling waters reached the second flood of one building at its height. One sight during the flood: a handmade, white flag hung from an upper story of a Main Street building bearing the letters SOS. Local officials recently said 96 percent of the businesses were back in operation and more than 20 new businesses had again opened in the Main Street area. The same area suffered massive flooding in 2016, when the city barely avoided having to tear down any buildings in a long and expensive rebuilding effort.