CHICAGO -- Chicago police are searching for the bus that dumped its waste on a boatload of Chicago tourists on Sunday.

As a boat full of tourists took in the sights on a picture-perfect day on the Chicago River, something fell down on them just as they got to the Kinzie Street Bridge.

And it wasn't rain, according to Chicago's WMAQ-TV. What fell down on the tourists through the bridge deck was raw sewage -- and it went all over the 120 passengers.

"A load of sewage was discarded from a tour bus that was over the top of the bridge above," said Lynn Osmond of the Chicago Architecture Foundation. "And it went all over our passengers."

WMAQ reported on Tuesday that two tour buses were reportedly on the Kinzie Street bridge at the time of the incident. Those tour buses might have belonged to a rock band. Investigators are trying to track down the specific bus in question.

One Chicago police sergeant told the television station that he went to an upscale hotel on North Michigan Avenue on Monday, and found the tour buses, and spoke to one of the drivers. That driver told police that the bus in question had already left the city.

The Environmental Protection Agency is investigating the incident.

Everyone involved is convinced that it was no accident, the television station reported.

Seven people went to Northwestern Hospital. Many people had to discard their clothes.

Authorities said they may be able to solve the mystery of who dumped the sewage on the tourists.

"We knew it was a large, black bus with no windows on it," said Anita Pedersen, director of public relations with Chicago's First Lady, the company that owns the boat. "They have a partial plate on it."

Pedersen added that this kind of thing has never happened in the 61-year history of the company.

The reason police have that partial plate number is that one of the sewage-covered tourists was able to jot down the number. Chicago police said they have traced it to a caravan of eight or nine buses that was apparently crossing the river at the time.