(CNN) Maryland Gov. Larry Hogan declared a state of emergency Sunday in Howard County as a massive storm drenched the Baltimore region, triggering flash floods in Ellicott City and leaving one person missing.

Thirty rescues were carried out into Monday morning, officials said.

Brown water rushed through Ellicott City's historic Main Street, toppling buildings and upending cars, as the nearby Patapsco River swelled to a record-breaking level. In some areas, water levels reached above the first floor of buildings, Howard County Fire and EMS said.

One person was missing following the Ellicott City flooding, Howard County Executive Allan Kittleman said Monday morning. The circumstances were not immediately known. No fatalities were reported.

The devastation was especially hard to comprehend coming barely two years after the last flood that ravaged the city, he said. The disaster left two dead and damaged dozens of buildings.

Kittleman said the flooding was worse than that endured in July 2016. Gas lines were shut down and a historic building fell, he told reporters.

The city rallied around the slogan "Elliott City Strong." Many businesses had just finished rebuilding, Kittleman said.

"There are no words," he said. "It's heartbreaking."

Advisories in effect

Ellicott City is an unincorporated community about 12 miles west of Baltimore. Located in the valley of the Patapsco River, a major waterway flowing to Chesapeake Bay, Ellicott City is known for its flood-prone location as much as its historic downtown.

The river rose 17.8 feet in two hours on Sunday afternoon to 24.13 feet, a new record from the previous high of 23.6 feet.

The National Weather Service issued a flash flood warning for northeastern Anne Arundel County and southeastern Howard County through earl Monday.

The NWS provided observed rainfall accumulations for the past 24 hours in the greater Baltimore area showing widespread observed amounts of 3 to 6 inches with isolated measurements exceeding 9 inches.

Additional rainfall amounts of 1 inch could come, the NWS said. That may not sound like a lot, but any additional rainfall will exacerbate the flooding from the rain that has already fallen, and this will delay river levels from receding below flood stage quickly, CNN's Allison Chinchar said.

Close to 8 p.m., a Howard County Sheriff's deputy moved a CNN crew and others in the area due to a suspected gas leak.

Multiple rescues are under way. Howard County Fire and EMS urged residents to evacuate downtown or move to higher ground while rescue teams swarmed the area.

The Roger Carter Community Center is open for those who need a place to shelter.

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Hogan toured the area Sunday with Kittleman. He lamented the destruction, noting that just two weeks earlier, he had visited the historic downtown and spoken with business owners about rebuilding efforts.

"The place looked terrific," he said. "It's just devastating because people have their lives tied up in this and went through a heck of a lot and came back and now they're starting all over again."

Kittleman said state and county resources would be made available to those who want to rebuild again.

He declined to answer a reporter's question about what could have been done differently to minimize the damage. The focus is on rescue and recovery, he said.

"Right now we're focusing on people's lives."