Hungarian authorities raced to finish building an emergency dam by Tuesday to hold back a threatened second spill of toxic sludge, and hunted for clues to the causes of last week’s deadly industrial spill.

A million cubic meters of red mud, a by-product of alumina production, burst out of a plant reservoir into villages and waterways in Hungary last Monday, killing seven people, injuring 123 and fouling rivers including a tributary of the Danube.

“We hope to have the dam finished by Tuesday,” Prime Minister Viktor Orban’s spokesman told TV2 on Monday.