Surrounded by big video screens and computers, a team works 24/7 to monitor the slightest threat of an accident at dams used by Brazil's Samarco iron ore mine.

The only problem? This impressive setup wasn't there when a huge dam burst open, killing 19 people 12 months ago.

Samarco, which is battling multi-billion dollar lawsuits and manslaughter charges, was keen to show journalists its high-tech safety gear ahead of the anniversary of the November 5, 2015 disaster.

The monitoring room near the town of Mariana is dominated by wall-to-wall screens with live footage of Samarco's dam installations, which hold back decades of mineral-packed sludge produced during the iron ore mining process.

A cutting edge radar system picks up any shifting in the structure of the dams, with seven employees sitting at computers, analyzing data around the clock.

And in an emergency, they'd go to a panel featuring big red buttons: evacuation sirens for workers and any inhabitants of communities downstream.

The siren at one town, Barra Longa, hasn't yet been linked up to the control room, but in the meantime Samarco has residents there covered too.

"There are trucks with loudspeakers on call 24 hours a day," emergencies coordinator Flavio Thimotio said.

- Too late -

But Brazilian prosecutors and survivors of last year's tragedy accuse Samarco, owned by Brazil's giant Vale company and Anglo-Australian BHP-Billiton, of shutting the stable door after the horse bolted.

The Fundao dam abruptly gave way, releasing 42 million cubic yards (32 million cubic meters) of mineral waste into the valley below, smashing the nearby village of Bento Rodrigues before continuing a path of destruction as far as the ocean, 400 miles (640 kilometers) away.

Jose Nascimento de Jesus, a 70-year-old inhabitant of Bento Rodrigues, says the only warning he got was the noise of the oncoming flood. "Like an airplane," he said.

"I was the last one to run out. In 10, 15 minutes it was all over."

Thimotio acknowledged that the monitoring operation at the time was a shadow of what it is today.

"They were installed after the event, with the great task of monitoring stability," he said of the video screens.

The radar system is also new. And many of the less sophisticated instruments Samarco previously relied on were not even in use on the day of the accident.

"They were switched off" for maintenance, Thimotio said.

As for the sirens, they didn't exist at all.

- Whose fault is it? -

A local state prosecutor in Mariana told AFP that he considers the lack of warning equipment "the worst thing."

"Sirens were obligatory as an alarm system for the population nearby and this system did not exist," the prosecutor, Guilherme de Sa Meneghin, said.

Announcing homicide charges against 21 people last month, including executives from Samarco, Vale and BHP, prosecutors claimed that "security was of secondary importance" to profits.

The three companies reject all the charges, insisting that the unnoticed weakening and eventual collapse of the Fundao dam was a freak, unstoppable accident.

"It was unprecedented," said Thimotio, stressing that in the run-up to the disaster instruments gave "no signal" of anything untoward.

The "Fundao dam was inspected regularly, not only by the authorities but also by independent international consultants," Samarco said in a statement.

"Safety has always been a priority in Samarco's management strategy, and the company reiterates that it never reduced its investments in safety."

Antonio Geraldo Santos, another Bento Rodrigues survivor, scoffs at these assurances.

"There was no warning, nothing," the 33-year-old said.

"People just ran for their lives."