The warning signs were already flashing as hundreds of young people celebrated the Fourth of July public holiday at Camp Mystic, an all-girls’ Christian summer retreat, nestled on the banks of the Guadalupe River in Texas.

There had not been a drop of rain in the area recently until the inundation, when the river rose 26ft (8m) in less than an hour, according to state officials.

By Saturday evening, at least 51 people were dead, including 15 children.

The first hint of the devastation to come appeared on Thursday morning as rain and thunderstorms soaked a number of central Texas counties.

The National Weather Service (NWS) issued a common warning called a flood watch at 13:18 that afternoon for parts of the region, including Kerr County.

In the early hours of Friday, the outlook became more dire as the NWS issued a series of upgraded warnings. The San Saba river, the Concho River and the Colorado River were rising.

At 04:03, the NWS sent a “particularly dangerous situation” alert, reserved for the most urgent and potentially deadly scenarios such as wildfires.

Another “particularly dangerous situation” warning was issued for the city of Kerrville at 05:34, before dawn on Friday.

“Residents and campers should SEEK HIGHER GROUND NOW! Life threatening flash flooding along the river is expected,” forecasters said.

“Automated rain gauges indicate a large and deadly flood wave is moving down the Guadalupe River. Flash flooding is already occurring.”

Such alerts are shared on NWS social media accounts and by broadcast news outlets, but most people were asleep.

Elinor Lester, 13, said younger campers at Camp Mystic were bunked in cabins closer to the riverbank and those were the first to flood.

“The camp was completely destroyed,” Elinor, who was evacuated by helicopter, told the Associated Press news agency. “It was really scary.”

Just outside Kerrville, the BBC met Jonathan and Brittany Rojas as they came to see what was left of a relative’s home. Only the foundations remain.

Five people were in the house the night of the deluge – the mother and her baby are still missing.

The teenage son, Leo, survived after he became snared in barbed wire, preventing him from being swept away. The boy is recovering in hospital.

As the BBC was interviewing the Rojas couple, a neighbour walked up to present them with an item salvaged from the house.

It was the teenager’s money jar. The label on it read, “Leo’s survival kit”.

Desperate Camp Mystic parents took to social media looking for news of their children.

One Facebook group – Kerrville Breaking News – turned into a missing persons page.

Some parents have since updated their social media pleas to say their missing family members did not survive.

Kerr County is in the heart of the Texas Hill Country, a getaway destination because of its scenic rolling hills, countless rivers and lakes and abundance of wineries.

But the region is also known as “Flash Flood Alley”, because of the recurring threat that has devastated local communities over the years.

When asked why the riverside summer camp was not evacuated, officials said the sudden scale of the deluge caught them unawares.

“No-one knew this kind of flood was coming,” Kerr County Judge Rob Kelly said.