Almost 800 rescued in St. John the Baptist Parish after Hurricane Ida

FEMA administrator Deanne Criswell called Ida "one of the most catastrophic hurricanes to make landfall" in the Gulf Coast region and said the storm left multiple collapsed buildings in its wake, particularly in the area of Baton Rouge.

"Some of the initial reports that we're hearing are some building collapses across the area, significant structural damage to many buildings," Creswell said Monday on "Good Morning America." "We're seeing some barges and some vessels that may have been broken loose and we're also experiencing over a million power outages right at the moment."

Criswell said emergency teams were heading out on search-and-rescue missions after receiving reports of people trapped in their homes by flooding.

"This is significant. There is major damage," Criswell said. "We've got a lot of resources in place to support the state."

Criswell added that "my biggest concern is still the fragility of our health care system."

"It has been stressed from COVID-19 and the occupancy has been full," said Criswell.

She said some hospitals in the hardest-hit areas of Louisiana were operating on backup generators.

Criswell said the good news is that it appears levees in southeast Louisiana, particularly those around New Orleans, held up through the storm.

"We knew there could be some in the southern parts of Louisiana that would overtop and those are areas that did have a mandatory evacuation order in place," Criswell said. "But we brought in search and rescue assets, power restoration teams, food and water to support the shelter operations."

The president of hard-hit Jefferson Parish, Louisiana, told ABC's "Good Morning America" Monday that all communication lines with Grand Isle were down.

Cynthia Lee Sheng said about 40 people are believed to have stayed on the barrier island, located about 100 miles south of New Orleans.

"We have lost contact with them since yesterday afternoon," Sheng said. "We have first responder teams out there planning their strategy for today, ready to go out."

Sheng also said there were concerns about Lafitte, Louisiana, saying officials had received reports of people trapped in their attics by high water.

"This is an area if you want to think of it like swampland, there's alligators out there," Sheng said.

She said rescue workers have not been able to reach the area due to darkness and downed power lines.

In addition to thousands in the area losing power, Sheng said the parish was losing pressure in its water system.

"We've had a lot of water main breaks," she said. "Our water system is losing pressure and so in order to be able to fight fires, that is a very critical element. So, we're trying to clear roads to do those water repairs."

Sheng added, "We're a broken community right now."

Ida, with its blustery winds and torrential rain, has left more than 1.1 million utility customers without power in Louisiana and Mississippi on Monday morning.

More than 1 million customers were without electricity in Louisiana, mostly in the southeast part of Bayou State where Ida made landfall, according to state emergency management officials.

In Mississippi, another 105,417 homes and businesses were without electricity, state officials said.

-ABC News' Will Gretsky

About 16 hours after making landfall in Louisiana, Ida was downgraded from a hurricane to a tropical storm early Monday morning.

As of 4 a.m. CT, Ida was moving north at 8 miles per hour with the eye of the storm located about 95 miles south-southwest of Jackson, Mississippi, and 50 miles north-northeast of Baton Rouge, Louisiana. The storm's maximum sustained winds have decreased near 60 miles per hour with higher gusts, according to an advisory from the National Weather Service.

The storm surge warning has been discontinued from Morgan City to Grand Isle, Louisiana. The hurricane and tropical storm warnings have been discontinued west of Grand Isle. The hurricane warning has been replaced with a tropical storm warning from Grand Isle to the mouth of the Pearl River, including Lake Pontchartrain, Lake Maurepas and metropolitan New Orleans. Storm surge and tropical storm warnings remain in effect for Grand Isle to the Alabama-Florida border, according to the National Weather Service.

Meanwhile, 16 states from Mississippi to New Jersey are still on alert for flash flooding. A flash flood watch is in place from the Gulf Coast to New Jersey.

So far, the highest rainfall total was recorded in LaPlace, Louisiana, which received 15 inches. A flash flood emergency remains in effect there, according to the National Weather Service.

Ida is forecast to rapidly weaken even more over the next day or so, becoming a tropical depression by Monday evening.

The storm will move farther inland over southeastern Louisiana early Monday and into southwestern Mississippi later in the morning. Ida is then forecast to move over central and northeastern Mississippi on Monday afternoon and evening before moving across the Tennessee Valley on Tuesday, according to the National Weather Service.

-ABC News' Max Golembo