Floodwaters reached the rooflines of single-story homes Monday and people could be heard pleading for help from inside as Harvey poured rain on the Houston area for a fourth consecutive day after a chaotic weekend of rising water and rescues.
The nation's fourth-largest city was still mostly paralyzed by one of the largest downpours in U.S. history. And there was no relief in sight from the storm that spun into Texas as a Category 4 hurricane, then parked over the Gulf Coast. With nearly 2 more feet (61 centimeters) of rain expected on top of the 30-plus inches (76 centimeters) in some places, authorities worried that the worst might be yet to come.
The storm system is expected to make landfall again sometime this week, forecasters told NBC News.
"This is a landmark event for Texas," said Brock Long, administrator of the Federal Emergency Management Agency. "Texas has never seen an event like this."
Harvey has been blamed for at least three confirmed deaths, including a woman killed Monday in the town of Porter, northeast of Houston, when a large oak tree dislodged by heavy rains toppled onto her trailer home.
A Houston television station reported Monday that six family members were believed to have drowned when their van was swept away by floodwaters. The KHOU report was attributed to three family members the station did not identify. No bodies have been recovered.
Police Chief Art Acevedo told The Associated Press that he had no information about the report but said that he's "really worried about how many bodies we're going to find."
According to the station, four children and their grandparents were feared dead after the van hit high water Sunday when crossing a bridge in the Greens Bayou area.
The driver of the vehicle, the children's great-uncle, reportedly escaped before the van sank by grabbing a tree limb. He told the children to try to escape through the back door, but they were unable to get out.
The disaster unfolded on an epic scale in one of America's most sprawling metropolitan centers. The Houston metro area covers about 10,000 square miles, an area slightly bigger than New Jersey. It's crisscrossed by about 1,700 miles of channels, creeks and bayous that drain into the Gulf of Mexico, about 50 miles to the southeast from downtown.
The storm is generating an amount of rain that would normally be seen only once in more than 1,000 years, said Edmond Russo, a deputy district engineer for the Army Corps of Engineers, which was concerned that floodwater would spill around a pair of 70-year-old reservoir dams that protect downtown Houston.
The flooding was so widespread that the levels of city waterways have equaled or surpassed those of Tropical Storm Allison from 2001, and no major highway has been spared some overflow.
The city's normally bustling business district was virtually deserted Monday, with emergency vehicles making up most of the traffic.
Rescuers continued plucking people from the floodwaters. Mayor Sylvester Turner put the number rescued by police at more than 3,000. The Coast Guard said it had also rescued more than 3,000 by boat and air and was taking more than 1,000 calls her hour.
Chris Thorn was among the many volunteers still helping with the mass evacuation that began Sunday. He drove with a buddy from the Dallas area with their flat-bottom hunting boat to pull strangers out of the water.
"I couldn't sit at home and watch it on TV and do nothing since I have a boat and all the tools to help," he said.
They got to Spring, Texas, where Cypress Creek had breached Interstate 45 and went to work, helping people out of a gated community near the creek.
"It's never flooded here," Lane Cross said from the front of Thorn's boat, holding his brown dog, Max. "I don't even have flood insurance."
A mandatory evacuation was ordered for the low-lying Houston suburb of Dickinson, home to 20,000. Police cited the city's fragile infrastructure in the floods, limited working utilities and concern about the weather forecast.
In Houston, questions continued to swirl about why the mayor did not issue a similar evacuation order.
Turner has repeatedly defended the decision and did so again Monday, insisting that a mass evacuation of millions of people by car was a greater risk than enduring the storm.
"Both the county judge and I sat down together and decided that we were not in direct path of the storm, of the hurricane, and the safest thing to do was for people to stay put, make the necessary preparations. I have no doubt that the decision we made was the right decision."
He added, "Can you imagine if millions of people had left the city of Houston and then tried to come back in right now?"
The Red Cross quickly set up the George R. Brown Convention Center and other venues as shelters. The center, which was also used to house Hurricane Katrina refugees from New Orleans in 2005, can accommodate roughly 5,000 people. By Monday evening, it was nearly full.
At the Addicks and Barker reservoirs, the Army Corps started releasing water Monday because water levels were climbing at a rate of more than 6 inches (15 centimeters) per hour, Corps spokesman Jay Townsend said.
The move was supposed to help shield the business district from floodwaters, but it also risked flooding thousands more homes in nearby subdivisions. Built after devastating floods in 1929 and 1935, the reservoirs were designed to hold water until it can be released downstream at a controlled rate.
In the Cypress Forest Estates neighborhood in northern Harris County, people called for help from inside homes as water from a nearby creek rose to their eaves. A steady procession of rescue boats floated into the area.
Harvey increased slightly in strength Monday as it drifted back over the warm Gulf, according to the National Hurricane Center.
Forecasters expect the system to stay over water with 45 mph (72 kph) winds for 36 hours and then head back inland east of Houston sometime Wednesday. The system will then head north and lose its tropical strength.
Before then, up to 20 more inches (51 centimeters) of rain could fall, National Weather Service Director Louis Uccellini said Monday.
That means the flooding will get worse in the days ahead and the floodwaters will be slow to recede once Harvey finally moves on, the weather service said.
Sometime Tuesday or early Wednesday, parts of the Houston region will probably break the nearly 40-year-old U.S. record for the biggest rainfall from a tropical system — 48 inches, set by Tropical Storm Amelia in 1978 in Texas, meteorologists said.
The amount of water in Houston was so unprecedented that the weather service on Wednesday had to update the color charts on its official rainfall maps to indicate the heavier totals.
In Louisiana, the images of the devastation in Houston are stirring up painful memories for many Hurricane Katrina survivors.
"It really evoked a lot of emotions and heartbreak for the people who are going through that now in Houston," Ray Gratia said as he picked up sandbags for his New Orleans home, which flooded during the 2005 hurricane.
The weather service issued flash flood watches through Thursday for southern parts of Louisiana, where the storm is expected to drop 10 to 15 more inches of rain.
On Monday, President Donald Trump declared an emergency in Lousiana and activated federal assistance for the state.
Trump's administration assured Congress that the $3 billion balance in the Federal Emergency Management Agency's disaster fund was enough to handle immediate needs, such as debris removal and temporary shelter for displaced residents. Trump planned to visit Texas on Tuesday.
Harvey was the fiercest hurricane to hit the U.S. in 13 years and the strongest to strike Texas since 1961's Hurricane Carla, the most powerful Texas hurricane on record.
Associated Press writers Juan Lozano and Nomaan Merchant in Houston and David Warren in Dallas contributed to this report.