- Video report by ITV News Washington Correspondent Robert Moore
Tropical Storm Harvey made landfall for a second time on Wednesday, with Louisiana joining Texas in facing life-threatening flooding.
The flood waters are beginning to drop across much of Houston, but the danger in Texas is not over, with many areas still submerged - including some emergency shelters.
The death toll from the storm has risen to at least 20, but the full scale of the disaster remains unclear, but many of those affected still unable to reach help.
Five days of torrential rain has submerged Houston, America's fourth-largest city and three major flood defences - one of which is designed to withstand a '1,000-year flood' - have overflown.
Houston's mayor, Sylvester Turner, imposed a night-time curfew between 10pm and 5am in a bid to prevent looting and protect public safety.
- The dramatic transformation of parts of Houston in 24 hours:
Around 24,000 members of the National Guard are being deployed in the state to help with disaster relief.
Mr Turner said he had asked the US Government to provide supplies, cots and food.
More than 17,000 people have sought refuge in Texas shelters and the American Red Cross said this figure was likely to increase.
The number of confirmed deaths rose to 20 when authorities reported that two men died in separate drownings. One of them drove around a barricade and into standing water on Monday, while the other tried to swim across a flooded road.
Texas Governor Greg Abbott said many of those who had died had done so after driving into flood waters and warned people not to do so.
Police said officers had found a submerged van in which a Houston family of six was killed as they tried to escape the floodwaters. The two grandparents and four children, aged six to 16, died after their van sank in Houston.
Authorities confirmed Houston police officer Sergeant Steve Perez, 60, drowned in his patrol car after he became trapped in high water while driving to work.
It has also been revealed that a three-year-old girl was found clinging to the body of her drowned mother, identified as 41-year-old Colette Sulcer, in a rain-swollen canal.
A woman was killed when heavy rain dislodged a large tree on to her trailer home in the small town of Porter and there are other reports of people missing or presumed dead.
A hotel in the city said one of its employees disappeared while helping about 100 guests and workers evacuate the building amid rising floodwaters.
Houston police chief Art Acevedo said: "I'm really worried about how many bodies we're going to find.
"We know in these kind of events that, sadly, the death toll goes up, historically."
Rescuers are continuing to pluck people from inundated neighbourhoods, with the National Guard carrying out or assisting in 3,500 rescues.
The Coast Guard said it was taking more than 1,000 calls per hour.
The storm made landfall again in south-western Louisiana in the early hours of Wednesday morning.
Donald Trump visited Texas on Tuesday for briefings on the government's work to help the state recover from Harvey, which has been downgraded from a hurricane to a tropical storm.
Addressing crowds on Tuesday, Mr Trump said: "This is historic, it's epic, what happened. But you know what? It happened in Texas and Texas can handle anything."
The US president then held a Texan flag aloft, sparking loud cheering from the crowd.
The storm marks the first time Mr Trump has been tested by a major natural disaster.
On Monday, he said "every asset at his command" was available to help those affected by the storm.
The president said that federal cash for storm-ravaged areas will arrive quickly.
"To the people of Texas and Louisiana, we are 100% with you," Mr Trump said.
Governor Abbott praised the president's response to the storm, saying that Mr Trump's commitment was “firm,strong and unequivocal” that he was going to do everything he could to ensure Texas will be restored to normality as soon as possible.
- ITV News Science Correspondent Alok Jha looks at the possible causes behind Storm Harvey