BUDAPEST - An aerial photo taken months before a gigantic reservoir unleashed torrents of toxic sludge shows a faint red trail trickling through the container wall -- part of a growing body of evidence that inspectors who gave the pit a clean bill of health may have missed warning signs.

Police were examining the photo Tuesday as part of an investigation into how part of the wall containing the 10 million cubic meters (350 million cubic feet) of caustic slurry could have given way without structural weaknesses being detected by a team of inspectors from the government environmental agency who inspected the container pond less then two weeks before the spill.

Disaster commissioner Gyorgy Bakondi, appointed to the newly created post Monday night, said Tuesday the inspections were under investigation, including claims by environmental inspectors that "they had found everything in order."

As the police probe gathered steam, judicial authorities scheduled a court appearance for Zoltan Bakonyi, the managing director of MAL Rt., or the Hungarian Aluminum Production and Trade Company, the company that owned the reservoir, to decide whether he should be formally charged, if so, with what, and whether he should remain in custody.

The photo showing an apparent leak of red sludge on the northern wall of the reservoir -- the same wall that partially collapsed eight days ago -- was taken by Interspect, a Hungarian company specializing in aerial photography that invests some of its profits on environmental projects, such as taking photos of locations in Hungary which could be at environmental risk.

Interspect director Gabor Bako said he shot the photo June 11, nearly 4 months before the spill. He said the company shared the photo with universities and environmental groups "but no further steps were taken in the matter" until the wall collapsed freeing the caustic muck that flooded three west Hungarian villages about 170 kilometres (just over 100 miles) from Budapest before being carried by local waterways into the Danube River.

Although Interspect found many suspicious sites around the country, "we're not construction engineers or specialists who could interpret what the picture showed," he told The Associated Press, still hoping to gather experts who could review the photos.

Bakonyi, the managing who was taken into police custody Monday, was scheduled to appear at a preliminary court hearing Wednesday convening at Veszprem, a western Hungarian city about 45 kilometres (27 miles) east of the partially collapsed containment pond.

A police statement issued Tuesday suggested Bakonyi was guilty of negligence, saying he did not prepare an emergency warning and rescue plan to be implemented in case of an incident like the sludge spill.

There was no official information on what Bakonyi told police, with law enforcement officials declining to divulge details on the progress of their investigation a week after the start of their probe. By Tuesday night, police had not made promised return calls to the AP.

But according to the daily Blikk, which is considered to have good police connections, a lead engineer at MAL Rt., told police that the firm's top management was aware -- but kept quiet -- about the risks of a breach of the reservoir for an unspecified period.

The tabloid also revealed that in the 1980s, before the fall of the Iron Curtain, Bakony's father, Arpad Bakonyi, was the head of the environmental department at the ministry of industry -- a predecessor of the present-day inspectorate -- and received several state awards for his work.

In an initial reaction after the spill, Zoltan Bakonyi said the reservoir was patrolled daily and "did not show any physical signs that something of this nature could happen." But Prime Minister Viktor Orban suggested that preliminary investigations revealed negligence playing a part.

"We have well-founded reasons to believe that there were people who knew about the dangerous weakening of the reservoir wall, but for personal reasons they thought it wasn't worth repairing and hoped there'd be no trouble," Orban said.

Bakondi, the disaster commissioner, said that police had taken over security tasks at all premises belonging to the company and that production at the plant could restart during the weekend, although a final decision had yet to be made.

Bakondi leads an 18-member supervisory committee, who will have to approve practically everything happening at MAL from now on.

The government rejected claims that the it was using the disaster as an excuse for ruling by decree.

"This is not the nationalization of the company," government spokeswoman Anna Nagy said. "It is placing it under government supervision until the catastrophe is resolved."

Asked, however, what activities the company could carry out without the consent of the supervisory board, Bakondi answered, "Nothing."

A corner of the reservoir at the alumina plant in Ajka, 160 kilometres (100 miles) southwest of Budapest, the capital, collapsed last Monday, releasing an estimated 700,000 cubic meters (184 million gallons) of a highly caustic byproduct of alumina production, which is then used to make aluminum.

The Ajkai Timfoldgyar plant, which began operating in 1943, was sold to private investors in the 1990s in the wake of the collapse of communism.

MAL has a 12 per cent market share in Europe of alumina production and 4 per cent globally.

It says it spent 30.3 billion forints ($153 million) in the past decade on maintenance and renovation work.

Media reports say it had revenues of nearly 29 billion forints ($147 million) in 2009 and 50 billion forints ($253 million) in 2007.