Multiple flash flood warnings remain in place over the weekend in central Texas.
About 850 people have been rescued so far.
At a news conference on Saturday afternoon, Texas Governor Greg Abbott said he had signed an expanded disaster declaration to boost search efforts.
He said officials would be relentless in ensuring they locate “every single person who’s been a victim of this event”, adding that “we will stop when the job is completed”.
It remains a search and rescue mission, officials said, not a recovery effort.
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They said rescuers were going up and down the Guadalupe River to try to find people who may have been swept away by the floods.
US President Donald Trump said his administration was working closely with local authorities to respond to the emergency.
Forecasters have warned that central Texas may see more flooding this weekend.
The National Weather Service (NWS) said the area could see 2 to 5in (5cm to 12cm) of rain on Saturday.
Up to 10in of rain was possible in some areas badly affected by Friday’s deluge.
Much of the rescue has focused on a large all-girls’ Christian summer camp called Camp Mystic, located along the banks of the Guadalupe River.
Texas Lieutenant Governor Dan Patrick told the BBC’s Radio 4 PM programme many of 27 missing girls were “under the age of 12”.
Pictures from the camp show it in disarray, with blankets, mattresses, teddy bears and other belongings caked in mud.
Many were asleep when the river rose more than 26ft (8m) in less than an hour in the early hours of Friday.
In an email to parents of the roughly 750 campers, Camp Mystic said that if they had not been contacted directly, their child had been accounted for.
More details of those who died have started to emerge – some were as young as eight. It has also been confirmed sisters Blair and Brooke Harber, who were 13 and 11, are among the dead.
A special Mass will be held at Notre Dame Catholic Church on Sunday for those who died or are missing, and their families.
Scores of campers holidaying during the Independence Day weekend, were also unaccounted for.
Lorena Guillen, whose home and restaurant were destroyed, had 28 holidaymakers’ cars staying on her land near the river. She told the BBC News Channel that she heard screams from a family of five.
“They were getting washed away,” Guillen said. “They were clinging on trees to be rescued. But the rescuers couldn’t get to them.”
Rachel Reed drove five hours from Dallas to pick up her daughter. She told the BBC that members of her Church and children’s school district were among the girls dead and missing.
“The families of those campers are living every parent’s worst nightmare,” she said. “Of course, it could have been me.”
Others started returning to the flooded areas.
Jonathan and Brittany Rojas visited their relatives’ home – where only the foundation remained.
They told the BBC that the mother and a baby of the family remained missing. A teenage son, Leo, survived after he became snared in barbed wire.
Another resident, Anthony, found his apartment full of mud and debris. His belongings were not salvageable, except a box holding childhood photos and his baby blanket.
“I lost everything I own,” he told the BBC. “Now I’m trying to figure things out.”
Additional reporting by Iona Hampson in Texas and Ana Faguy in Washington.