CEDAR RAPIDS, Iowa (CNN) -- Thousands of people didn't know Monday when they would be able to return to their flooded eastern Iowa homes.
In Cedar Rapids, residents had been allowed to return home temporarily to retrieve keepsakes and other items from their homes Sunday, but authorities said Monday that strike teams had determined the neighborhoods were no longer safe, even for a quick visit.
"We are taking a step back," Cedar Rapids Fire Department spokesman Dave Brown said, saying it would be awhile before evacuees would be permitted to go back home.
Police set up checkpoints to keep people away from the affected neighborhoods, deemed unsafe after weeks of heavy rain forced the Cedar River from its banks, leaving much of Iowa's second-largest city under water.
waited in line at the checkpoints Sunday to receive special wristbands that allowed them to go home and gather their belongings. Authorities set a curfew and asked the residents to stay out of the neighborhoods between 8 p.m. and 6 a.m., so people stuffed what they could into plastic bags and returned to the checkpoints.
Tracy Murphy made a beeline for her family photos when she entered her house, parts of which looked as if burglars had ransacked it.
"Anything can be replaced, but your photos can't," she said.
A trash can was overturned and belongings were strewn across rooms. Murphy's eyes welled up as the realization hit her: "My whole, entire life is gone."
The checkpoints -- manned by police and the Iowa National Guard -- remained in place Monday, but Brown said authorities would not be letting residents check on their homes.
Strike teams assessed the residential and commercial areas where the waters had receded Sunday and determined those areas were not safe for re-entry, he said.
Veronica Johnson evacuated her home and later had to evacuate her mother's house, she said. She hasn't been able to return in four days -- and not just because of dangerous floodwaters.
"I live by a gas station, and the tankers from underground busted up through the ground, so I have gas spilled all around my house and the whole neighborhood," she said.
Residents have been getting angry with the authorities who are keeping them from their homes, she said, but she understands safety comes first.
"They have Red Cross, police department, fire department, and the people who they brought in -- the Marines and stuff, the National Guard -- have been excellent," she said. "They are keeping us out of our homes even though we're getting upset with them. We have no right because they're trying to protect us."
Local authorities expect to release a list of areas that are safe by Monday afternoon, with the hope that people can begin returning to their homes Tuesday, Brown said.
About 36,000 Iowans were evacuated because of statewide flooding, 24,000 of them in Cedar Rapids. The massive flooding has overwhelmed the city -- which is in a 500-year flood plain, an area the federal government says has less than a 0.2 percent chance of flooding.
"It's been compared to a 3,000-year flood," Cedar Rapids police Detective Brad Novak said. "So something with that rarity of an event, there is no playbook to go by."
Iowa Gov. Chet Culver has declared 83 of the state's 99 counties disaster areas. More than 3,300 Iowa National Guard troops have been deployed to help primarily with sandbagging and staging resources, Maj. Gen. Ron Dardis of the Iowa National Guard said Sunday. Another 700 troops were expected to join them Monday.
Culver estimates agricultural damage could reach $1 billion, exceeding the costs of the big flood in 1993.
This month's severe weather has trampled towns from North Dakota to Indiana. The Federal Emergency Management Agency says more than 11 million Midwesterners will be affected by flooding and tornadoes.
Iowa has been inundated with heavy rains in recent weeks that have caused several major rivers that feed the Mississippi -- including the Cedar, Des Moines and Iowa rivers -- to flood their banks.
The flooding is "some of the worst" to hit the United States since Hurricane Katrina slammed into the Gulf Coast nearly three years ago, FEMA Administrator David Paulison said Sunday.
The agency has received more than 12,000 disaster assistance applications from the hardest-hit states -- Iowa, Indiana and Wisconsin.
Starting Tuesday, the American Red Cross will set up kitchens in Iowa to serve up about 100,000 meals to residents each day. The agency, which is housing 720 flood victims in 30 shelters, plans to spend about $15 million on Midwest relief efforts.