US rock group The Dave Matthews Band have been accused of dumping human waste from a bus into a river, dousing a tour boat filled with passengers.
The Dave Matthews Band will "co-operate" in the investigation
More than 100 people on an architecture tour were showered with foul-smelling sewage in the incident on 8 August.
The driver of a bus leased by the band allegedly emptied the septic tank through a grate in the bridge and into the Chicago River below.
The band are being sued by the state of Illinois for violating state laws.
The state is seeking $70,000 (£39,000) in civil penalties against the band and one of its drivers for allegedly violating water pollution and public nuisance legislation.
The state claims up to 800lb (360kg) of liquid human waste was released from the bus.
John Vlautin, spokesman for The Dave Matthews Band, said: "Our driver has stated that he was not involved in this incident."
He added the band "will continue to be co-operative in this investigation".
According to the lawsuit, a bus leased by the band was heading to a hotel when the driver allegedly emptied the contents of the septic tank as it crossed the Kinzie Street bridge.
None of the people on the tour boat were seriously injured in the incident and they were all refunded for the trip. The boat was cleaned with disinfectant.
Attorney General Lisa Madigan said: "This incident may be unique, but that does not lessen the environmental or public health risks posed by the release of at least 800 pounds of liquid human waste into a busy waterway and onto a crowded tour boat."
The Dave Matthews Band have a huge following in the US where they built their reputation as a live band.
They won a Grammy Award in 1996 for their single So Much To Say and are currently in the middle of a US tour.