(AP) -- Hungary opened a criminal probe into the toxic sludge flood Wednesday and the European Union urged emergency authorities to do everything they can to keep the contaminated slurry from reaching the Danube and affecting half a dozen other nations.
Hundreds of people had to be evacuated after a gigantic sludge reservoir burst Monday at a metals plant in Ajka, a town 100 miles (160 kilometers) southwest of Budapest, the capital.
At least four people were killed, three are still missing and 120 were injured as the unstoppable torrent inundated homes, swept cars off roads and disgorged an estimated 1 million cubic meters (35 million cubic feet) of toxic waste onto several nearby towns.
It was still not known Wednesday why part of the reservoir failed. Hungarian Prime Minister Viktor Orban said authorities were caught off guard by the disaster since the plant and reservoir had been inspected only two weeks earlier and no irregularities had been found.
National Police Chief Jozsef Hatala decided to take over the probe because of its importance and complexity, police spokeswoman Monika Benyi told The Associated Press, adding that a criminal case had been opened by the country's top investigative body into possible on-the-job carelessness.
The huge reservoir, more than 1,000 feet (300 meters) long and 500 yards (450 meters) wide, was no longer leaking Wednesday but a triple-tiered protective wall was being built around its damaged area. Interior Minister Sandor Pinter said guards have been posted at the site to give an early warning in case of any new emergency.
The red torrent has already reached the Marcal River but it was not clear Wednesday how far down the river it had spread. Emergency workers were pouring 1,000 tons of plaster into the water to try to bind the sludge and keep it from flowing into the Danube, 45 miles (72 kilometers) away.
The Hungarian Water Regulation Authority estimated Tuesday it would take the sludge about five days to reach the Danube, one of Europe's key waterways. South of Hungary, the Danube flows through Croatia, Serbia, Romania, Bulgaria, Ukraine and Moldova before emptying into the Black Sea.
Hungary's National Rescue Service said engineers considered diverting the Marcal into nearby fields but decided not to, fearing the damage from the diversion would be too great.
Workers were also extracting sludge from the river and using plaster and acid to neutralize the toxic chemicals. Initial pH measurements showed the sludge had an extremely alkaline value of 13 after the spill, the service said.
The European Union said it feared the toxic flood could turn into an ecological disaster for several nations and urged Hungarian authorities to focus all efforts on keeping the sludge from the Danube.
"It is important that we do .... everything possible that it would not go, that it would not endanger the Danube," EU Environment Commissioner Janez Potocnik told the AP in Brussels. "We have to do this very moment everything possible ... (to) limit the extent of the damage."
"This is a serious environmental problem," EU spokesman Joe Hennon told Associated Press Television News. "We are concerned, not just for the environment in Hungary, but this could potentially cross borders."
Greenpeace was even more emphatic.
The sludge spill is "one of the top three environmental disasters in Europe in the last 20 or 30 years," said Herwit Schuster, a spokesman for Greenpeace International.
Greenpeace workers took sludge samples on Tuesday and were having them tested in labs in Vienna and Budapest to find out how contaminated the sludge was by heavy metals.
"It is clear that 40 sq. kilometers (15.5 square miles) of mostly agricultural land is polluted and destroyed for a long time," Schuster said. "If there are substances like arsenic and mercury, that would affect river systems and ground water on long-term basis."
Red sludge is a byproduct of the refining of bauxite into alumina, the basic material for manufacturing aluminum. It contains heavy metals and is toxic if ingested. Treated sludge is often stored in ponds where the water eventually evaporates, leaving behind a dried red clay-like soil.
MAL Rt., the Hungarian Aluminum Production and Trade Company that owns the Ajkai plant, has insisted the red sludge is not considered hazardous waste according to EU standards. The company has also rejected criticism that it should have taken more precautions to shore up the reservoir.
In Hungary's hardest-hit towns, emergency workers and construction crews in respirators and other hazmat gear strained Wednesday to clear roads and homes coated by thick red sludge and caustic muddy water.
In Kolontar, the town nearest to the plant, a military construction crew assembled a pontoon bridge across a toxic stream so residents could briefly return to their homes and retrieve some belongings.
But Kolontar mayor Karoly Tily said he could not reassure residents that Monday's calamity would not happen again.
In sharp contrast to the emergency workers, locals salvaged possessions with little more than rubber gloves for protection. Women with pants coated with red mud cleared the muck away from their homes with snow shovels.
The International Commission for the Protection of the Danube, which manages the river and its tributaries, agreed that sludge spill could trigger long-term damaging effects for both wildlife and humans.
"It is a very serious accident and has potential implications for other countries," Philip Weller, the group's executive secretary, said from Brussels.
The Danube, at 1,775 miles (2,850 kilometers) long, is Europe's second largest river and holds one of the continent's greatest treasuries of wildlife. The river has already been the focus of a multibillion dollar post-communist cleanup, but high-risk industries such as Hungary's Ajkai Timfoldgyar alumina plant, where the disaster occurred, are still producing waste near some of its tributaries.
Weller said the commission's early warning alarm system was triggered by the spill, which means factories and towns along the Danube may have to shut down their water intake systems. The Vienna-based commission was waiting for further details of the spill from Hungarian authorities, he said.
He said large fish in the Danube could ingest the metals and then transfer them to humans who eat the fish.
The ecological catastrophe has already left a trail of shattered lives in its wake.
There was no stopping the avalanche of toxic red sludge when it rammed into Kati Holtzer's home in Kolontar: It smashed through the main door and trapped the woman and her 3-year-old boy in a churning sea of acrid waste.
She saved her son by placing him on a sofa that was floating in the muck. She then called her husband Balazs, who was working in Austria, to say goodbye.
"We're going to die," she told him, chest-deep in sludge.
After the terror came the pain: Holtzer and her two rescuers were among those suffering from biting chemical burns. Half the house was painted red from the sludge.
Worst of all, her fox terrier Mazli - "Luck" in Hungarian - lay dead in the yard Wednesday, still chained to a stake.
Explore further: Mexico sets 25 pct pollution cut by 2030 for climate talks