Passengers aboard Chicago's Little Lady architecture tour Sunday say they saw a large tour bus dump liquid waste off the Kinzie Street Bridge, just as the boat cruised underneath.
More than 100 passengers were on the boat when the waste poured onto the open upper deck.
Daley called the incident "unacceptable" and is urging anyone with information about the incident to come forward.
The Attorney General's Office started its own investigation into whether any state environmental laws were broken.
State investigators and a lawyer interviewed the boat crew Tuesday.
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