Minnesota officials on Monday were trying to determine what killed thousands of fish in Lake Owasso. There’s no evidence of a chemical spill or toxins, and tests show that oxygen levels are normal, said Harland Hiemstra, a spokesman for the Minnesota Department of Natural Resources. Lake Owasso lies within Shoreview and Roseville in the northern St. Paul suburbs. The lake is managed primarily for muskies, he said. Lake resident Les Hassler said he’s counted thousands of dead fish since it froze over last Tuesday. Hundreds of fish could be seen belly-up inches beneath the new ice on Sunday.